Add a testing tool chain

- use pytest as the base, measure coverage with pytest-cov
  + configure coverage to include branches and specify source locations
  + configure pytest to enforce explicit markers
- add a package for the test suite under tests/
- add a `__version__` identifier at the package's root
  + it is dynamically assigned the version of the installed package
  + the version is PEP440 compliant and follows a strict subset of
    semantic versioning: x.y.z[.devN] where x, y, z, and N are all
    non-negative integers
  + add module with tests for the __version__
- add a nox session "test" that runs the test suite
- use flake8 to lint pytest for consistent style

setup.cfg

@@ -2,6 +2,23 @@
 # black's settings are in pyproject.toml => [tool.black]
 
 
+[coverage:paths]
+source =
+    src/
+    */site-packages/
+
+[coverage:report]
+show_missing = true
+skip_covered = true
+skip_empty = true
+
+[coverage:run]
+branch = true
+data_file = .cache/coverage/data
+source =
+    urban_meal_delivery
+
+
 [flake8]
 # Include error classes only explicitly
 # to avoid forward compatibility issues.
@@ -57,6 +74,8 @@ select =
     BLK1, BLK9,
     # flake8-expression-complexity => not too many expressions at once
     ECE001,
+    # flake8-pytest-style => enforce a consistent style with pytest
+    PT0,
 
 # By default, flake8 ignores some errors.
 # Instead, do not ignore anything.
@@ -70,6 +89,10 @@ extend-ignore =
     E203, W503,
     # f-strings are ok.
     WPS305,
+    # Classes should not have to specify a base class.
+    WPS306,
+    # Putting logic into __init__.py files may be justified.
+    WPS412,
 
 per-file-ignores =
     noxfile.py:
@@ -79,6 +102,11 @@ per-file-ignores =
         WPS213,
         # No overuse of string constants (e.g., '--version').
         WPS226,
+    tests/*.py:
+        # `assert` statements are ok in the test suite.
+        S101,
+        # Shadowing outer scopes occurs naturally with mocks.
+        WPS442,
 
 # Explicitly set mccabe's maximum complexity to 10 as recommended by
 # Thomas McCabe, the inventor of the McCabe complexity, and the NIST.
@@ -106,6 +134,17 @@ docstring-convention = google
 # flake8-eradicate
 eradicate-aggressive = true
 
+# ==============================
+# flake8-pytest-style's settings
+# ==============================
+# Prefer @pytest.fixture over @pytest.fixture().
+pytest-fixture-no-parentheses = true
+# Prefer @pytest.mark.parametrize(['param1', 'param2'], [(1, 2), (3, 4)])
+# over @pytest.mark.parametrize(('param1', 'param2'), ([1, 2], [3, 4]))
+pytest-parametrize-names-type = list
+pytest-parametrize-values-row-type = tuple
+pytest-parametrize-values-type = list
+
 
 [isort]
 atomic = true
@@ -134,7 +173,7 @@ single_line_exclusions = typing
 [mypy]
 cache_dir = .cache/mypy
 
-[mypy-nox.*]
+[mypy-nox.*,packaging.*,pytest]
 ignore_missing_imports = true
 
 
@@ -165,6 +204,15 @@ disable =
     missing-class-docstring, missing-function-docstring, missing-module-docstring,
     # pyflakes
     undefined-variable, unused-import, unused-variable,
+    # wemake-python-styleguide
+    redefined-outer-name,
 
 [pylint.REPORTS]
 score = no
+
+
+[tool:pytest]
+addopts =
+    --strict-markers
+cache_dir = .cache/pytest
+console_output_style = count