Add an ORM layer

- use SQLAlchemy (and PostgreSQL) to model the ORM layer
- add the following models:
  + Address => modelling all kinds of addresses
  + City => model the three target cities
  + Courier => model the UDP's couriers
  + Customer => model the UDP's customers
  + Order => model the orders received by the UDP
  + Restaurant => model the restaurants active on the UDP
- so far, the emphasis lies on expression the Foreign Key
  and Check Constraints that are used to validate the assumptions
  inherent to the cleanded data
- provide database-independent unit tests with 100% coverage
- provide additional integration tests ("e2e") that commit data to
  a PostgreSQL instance to validate that the constraints work
- adapt linting rules a bit

setup.cfg

@@ -84,6 +84,8 @@ ignore =
 # If --ignore is passed on the command
 # line, still ignore the following:
 extend-ignore =
+    # Too long line => duplicate with E501.
+    B950,
     # Comply with black's style.
     # Source: https://github.com/psf/black/blob/master/docs/compatible_configs.md#flake8
     E203, W503,
@@ -114,6 +116,10 @@ per-file-ignores =
         WPS115,
         # Numbers are normal in config files.
         WPS432,
+    src/urban_meal_delivery/db/addresses.py:
+        WPS226,
+    src/urban_meal_delivery/db/orders.py:
+        WPS226,
     tests/*.py:
         # Type annotations are not strictly enforced.
         ANN0, ANN2,
@@ -121,8 +127,12 @@ per-file-ignores =
         S101,
         # Shadowing outer scopes occurs naturally with mocks.
         WPS442,
+        # Modules may have many test cases.
+        WPS202,WPS204,WPS214,
         # No overuse of string constants (e.g., '__version__').
         WPS226,
+        # Numbers are normal in test cases as expected results.
+        WPS432,
 
 # Explicitly set mccabe's maximum complexity to 10 as recommended by
 # Thomas McCabe, the inventor of the McCabe complexity, and the NIST.
@@ -199,6 +209,8 @@ ignore_missing_imports = true
 ignore_missing_imports = true
 [mypy-pytest]
 ignore_missing_imports = true
+[mypy-sqlalchemy.*]
+ignore_missing_imports = true
 
 
 [pylint.FORMAT]
@@ -210,6 +222,9 @@ disable =
     # We use TODO's to indicate locations in the source base
     # that must be worked on in the near future.
     fixme,
+    # Too many false positives and cannot be disabled within a file.
+    # Source: https://github.com/PyCQA/pylint/issues/214
+    duplicate-code,
     # Comply with black's style.
     bad-continuation, bad-whitespace,
     # =====================
@@ -240,3 +255,5 @@ addopts =
     --strict-markers
 cache_dir = .cache/pytest
 console_output_style = count
+markers =
+    e2e: integration tests, inlc., for example, tests touching a database