Add database migrations

- use Alembic to migrate the PostgreSQL database
  + create initial migration script to set up the database,
    as an alternative to db.Base.metadata.create_all()
  + integrate Alembic into the test suite; the db_engine fixture
    now has two modes:
    * create the latest version of tables all at once
    * invoke `alembic upgrade head`
    => the "e2e" tests are all run twice, once in each mode; this
       ensures that the migration scripts re-create the same database
       schema as db.Base.metadata.create_all() would
    * in both modes, a temporary PostgreSQL schema is used to create the
      tables in
    => could now run "e2e" tests against production database and still
       have isolation
- make the configuration module public (to be used by Alembic)
- adjust linting rules for Alembic

poetry.lock

@@ -6,6 +6,20 @@ optional = false
 python-versions = "*"
 version = "0.7.12"
 
+[[package]]
+category = "main"
+description = "A database migration tool for SQLAlchemy."
+name = "alembic"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
+version = "1.4.2"
+
+[package.dependencies]
+Mako = "*"
+SQLAlchemy = ">=1.1.0"
+python-dateutil = "*"
+python-editor = ">=0.3"
+
 [[package]]
 category = "dev"
 description = "A small Python module for determining appropriate platform-specific dirs, e.g. a \"user data dir\"."
@@ -562,7 +576,22 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 version = "1.4.3"
 
 [[package]]
-category = "dev"
+category = "main"
+description = "A super-fast templating language that borrows the  best ideas from the existing templating languages."
+name = "mako"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
+version = "1.1.3"
+
+[package.dependencies]
+MarkupSafe = ">=0.9.2"
+
+[package.extras]
+babel = ["babel"]
+lingua = ["lingua"]
+
+[[package]]
+category = "main"
 description = "Safely add untrusted strings to HTML/XML markup."
 name = "markupsafe"
 optional = false
@@ -812,6 +841,28 @@ pytest = ">=4.6"
 [package.extras]
 testing = ["fields", "hunter", "process-tests (2.0.2)", "six", "pytest-xdist", "virtualenv"]
 
+[[package]]
+category = "dev"
+description = "py.test plugin that allows you to add environment variables."
+name = "pytest-env"
+optional = false
+python-versions = "*"
+version = "0.6.2"
+
+[package.dependencies]
+pytest = ">=2.6.0"
+
+[[package]]
+category = "main"
+description = "Extensions to the standard Python datetime module"
+name = "python-dateutil"
+optional = false
+python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,>=2.7"
+version = "2.8.1"
+
+[package.dependencies]
+six = ">=1.5"
+
 [[package]]
 category = "main"
 description = "Add .env support to your django/flask apps in development and deployments"
@@ -823,6 +874,14 @@ version = "0.14.0"
 [package.extras]
 cli = ["click (>=5.0)"]
 
+[[package]]
+category = "main"
+description = "Programmatically open an editor, capture the result."
+name = "python-editor"
+optional = false
+python-versions = "*"
+version = "1.0.4"
+
 [[package]]
 category = "dev"
 description = "World timezone definitions, modern and historical"
@@ -877,7 +936,7 @@ version = "1.3.1"
 docutils = ">=0.11,<1.0"
 
 [[package]]
-category = "dev"
+category = "main"
 description = "Python 2 and 3 compatibility utilities"
 name = "six"
 optional = false
@@ -1179,7 +1238,7 @@ optional = ["pygments", "colorama"]
 tests = ["pytest", "pytest-cov", "codecov", "scikit-build", "cmake", "ninja", "pybind11"]
 
 [metadata]
-content-hash = "508cbaa3105e47cac64c68663ed8d4178ee752bf267cb24cf68264e73325e10b"
+content-hash = "3227fd9a5706b1483adc9b6cb7350515ffda05c38ab9c9a83d63594b3f4f6673"
 lock-version = "1.0"
 python-versions = "^3.8"
 
@@ -1188,6 +1247,9 @@ alabaster = [
     {file = "alabaster-0.7.12-py2.py3-none-any.whl", hash = "sha256:446438bdcca0e05bd45ea2de1668c1d9b032e1a9154c2c259092d77031ddd359"},
     {file = "alabaster-0.7.12.tar.gz", hash = "sha256:a661d72d58e6ea8a57f7a86e37d86716863ee5e92788398526d58b26a4e4dc02"},
 ]
+alembic = [
+    {file = "alembic-1.4.2.tar.gz", hash = "sha256:035ab00497217628bf5d0be82d664d8713ab13d37b630084da8e1f98facf4dbf"},
+]
 appdirs = [
     {file = "appdirs-1.4.4-py2.py3-none-any.whl", hash = "sha256:a841dacd6b99318a741b166adb07e19ee71a274450e68237b4650ca1055ab128"},
     {file = "appdirs-1.4.4.tar.gz", hash = "sha256:7d5d0167b2b1ba821647616af46a749d1c653740dd0d2415100fe26e27afdf41"},
@@ -1435,6 +1497,10 @@ lazy-object-proxy = [
     {file = "lazy_object_proxy-1.4.3-cp38-cp38-win32.whl", hash = "sha256:5541cada25cd173702dbd99f8e22434105456314462326f06dba3e180f203dfd"},
     {file = "lazy_object_proxy-1.4.3-cp38-cp38-win_amd64.whl", hash = "sha256:59f79fef100b09564bc2df42ea2d8d21a64fdcda64979c0fa3db7bdaabaf6239"},
 ]
+mako = [
+    {file = "Mako-1.1.3-py2.py3-none-any.whl", hash = "sha256:93729a258e4ff0747c876bd9e20df1b9758028946e976324ccd2d68245c7b6a9"},
+    {file = "Mako-1.1.3.tar.gz", hash = "sha256:8195c8c1400ceb53496064314c6736719c6f25e7479cd24c77be3d9361cddc27"},
+]
 markupsafe = [
     {file = "MarkupSafe-1.1.1-cp27-cp27m-macosx_10_6_intel.whl", hash = "sha256:09027a7803a62ca78792ad89403b1b7a73a01c8cb65909cd876f7fcebd79b161"},
     {file = "MarkupSafe-1.1.1-cp27-cp27m-manylinux1_i686.whl", hash = "sha256:e249096428b3ae81b08327a63a485ad0878de3fb939049038579ac0ef61e17e7"},
@@ -1580,10 +1646,24 @@ pytest-cov = [
     {file = "pytest-cov-2.10.0.tar.gz", hash = "sha256:1a629dc9f48e53512fcbfda6b07de490c374b0c83c55ff7a1720b3fccff0ac87"},
     {file = "pytest_cov-2.10.0-py2.py3-none-any.whl", hash = "sha256:6e6d18092dce6fad667cd7020deed816f858ad3b49d5b5e2b1cc1c97a4dba65c"},
 ]
+pytest-env = [
+    {file = "pytest-env-0.6.2.tar.gz", hash = "sha256:7e94956aef7f2764f3c147d216ce066bf6c42948bb9e293169b1b1c880a580c2"},
+]
+python-dateutil = [
+    {file = "python-dateutil-2.8.1.tar.gz", hash = "sha256:73ebfe9dbf22e832286dafa60473e4cd239f8592f699aa5adaf10050e6e1823c"},
+    {file = "python_dateutil-2.8.1-py2.py3-none-any.whl", hash = "sha256:75bb3f31ea686f1197762692a9ee6a7550b59fc6ca3a1f4b5d7e32fb98e2da2a"},
+]
 python-dotenv = [
     {file = "python-dotenv-0.14.0.tar.gz", hash = "sha256:8c10c99a1b25d9a68058a1ad6f90381a62ba68230ca93966882a4dbc3bc9c33d"},
     {file = "python_dotenv-0.14.0-py2.py3-none-any.whl", hash = "sha256:c10863aee750ad720f4f43436565e4c1698798d763b63234fb5021b6c616e423"},
 ]
+python-editor = [
+    {file = "python-editor-1.0.4.tar.gz", hash = "sha256:51fda6bcc5ddbbb7063b2af7509e43bd84bfc32a4ff71349ec7847713882327b"},
+    {file = "python_editor-1.0.4-py2-none-any.whl", hash = "sha256:5f98b069316ea1c2ed3f67e7f5df6c0d8f10b689964a4a811ff64f0106819ec8"},
+    {file = "python_editor-1.0.4-py2.7.egg", hash = "sha256:ea87e17f6ec459e780e4221f295411462e0d0810858e055fc514684350a2f522"},
+    {file = "python_editor-1.0.4-py3-none-any.whl", hash = "sha256:1bf6e860a8ad52a14c3ee1252d5dc25b2030618ed80c022598f00176adc8367d"},
+    {file = "python_editor-1.0.4-py3.5.egg", hash = "sha256:c3da2053dbab6b29c94e43c486ff67206eafbe7eb52dbec7390b5e2fb05aac77"},
+]
 pytz = [
     {file = "pytz-2020.1-py2.py3-none-any.whl", hash = "sha256:a494d53b6d39c3c6e44c3bec237336e14305e4f29bbf800b599253057fbb79ed"},
     {file = "pytz-2020.1.tar.gz", hash = "sha256:c35965d010ce31b23eeb663ed3cc8c906275d6be1a34393a1d73a41febf4a048"},