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

src/urban_meal_delivery/__init__.py

@@ -1 +1,9 @@
 """Source code for the urban-meal-delivery research project."""
+
+from importlib import metadata
+
+
+try:
+    __version__ = metadata.version(__name__)
+except metadata.PackageNotFoundError:  # pragma: no cover
+    __version__ = 'unknown'