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

tests/__init__.py

@@ -0,0 +1 @@
+"""Test the urban-meal-delivery package."""

tests/test_version.py

@@ -0,0 +1,116 @@
+"""Test the package's version identifier.
+
+This packaged version identifier must adhere to PEP440
+and a strict subset of semantic versioning:
+
+- version identifiers must follow the x.y.z format
+  where x.y.z are non-negative integers
+- non-final versions are only allowed with a .devN suffix
+  where N is also a non-negative integer
+"""
+
+import re
+
+import pytest
+from packaging import version as pkg_version
+from packaging.version import Version
+
+import urban_meal_delivery
+
+
+@pytest.fixture
+def parsed_version() -> str:
+    """The packaged version."""
+    return pkg_version.Version(urban_meal_delivery.__version__)  # noqa:WPS609
+
+
+class TestPEP404Compliance:
+    """Packaged version identifier is PEP440 compliant."""
+
+    # pylint:disable=no-self-use
+
+    def test_parsed_version_has_no_epoch(self, parsed_version: Version) -> None:
+        """PEP440 compliant subset of semantic versioning: no epoch."""
+        assert parsed_version.epoch == 0
+
+    def test_parsed_version_is_non_local(self, parsed_version: Version) -> None:
+        """PEP440 compliant subset of semantic versioning: no local version."""
+        assert parsed_version.local is None
+
+    def test_parsed_version_is_no_post_release(self, parsed_version: Version) -> None:
+        """PEP440 compliant subset of semantic versioning: no post releases."""
+        assert parsed_version.is_postrelease is False
+
+    def test_parsed_version_is_all_public(self, parsed_version: Version) -> None:
+        """PEP440 compliant subset of semantic versioning: all public parts."""
+        assert parsed_version.public == urban_meal_delivery.__version__  # noqa:WPS609
+
+
+class TestSemanticVersioning:
+    """Packaged version follows a strict subset of semantic versioning."""
+
+    # pylint:disable=no-self-use
+
+    version_pattern = re.compile(
+        r'^(0|([1-9]\d*))\.(0|([1-9]\d*))\.(0|([1-9]\d*))(\.dev(0|([1-9]\d*)))?$',
+    )
+
+    def test_version_is_semantic(self) -> None:
+        """Packaged version follows semantic versioning."""
+        result = self.version_pattern.fullmatch(
+            urban_meal_delivery.__version__,  # noqa:WPS609
+        )
+
+        assert result is not None
+
+    # The next two test cases are sanity checks to validate the version_pattern.
+
+    @pytest.mark.parametrize(
+        'version',
+        [
+            '0.1.0',
+            '1.0.0',
+            '1.2.3',
+            '1.23.456',
+            '123.4.56',
+            '10.11.12',
+            '1.2.3.dev0',
+            '1.2.3.dev1',
+            '1.2.3.dev10',
+        ],
+    )
+    def test_valid_semantic_versioning(self, version: str) -> None:
+        """Versions follow the x.y.z or x.y.z.devN format."""
+        result = self.version_pattern.fullmatch(version)
+
+        assert result is not None
+
+    @pytest.mark.parametrize(
+        'version',
+        [
+            '1',
+            '1.2',
+            '-1.2.3',
+            '01.2.3',
+            '1.02.3',
+            '1.2.03',
+            '1.2.3.4',
+            '1.2.3.abc',
+            '1.2.3-dev0',
+            '1.2.3+dev0',
+            '1.2.3.d0',
+            '1.2.3.develop0',
+            '1.2.3.dev-1',
+            '1.2.3.dev01',
+            '1.2.3..dev0',
+            '1-2-3',
+            '1,2,3',
+            '1..2.3',
+            '1.2..3',
+        ],
+    )
+    def test_invalid_semantic_versioning(self, version: str) -> None:
+        """Versions follow the x.y.z or x.y.z.devN format."""
+        result = self.version_pattern.fullmatch(version)
+
+        assert result is None