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

pyproject.toml

@@ -41,6 +41,12 @@ flake8 = "^3.8.3"
 flake8-annotations = "^2.3.0"
 flake8-black = "^0.2.1"
 flake8-expression-complexity = "^0.0.8"
+flake8-pytest-style = "^1.2.2"
 mypy = "^0.782"
 pylint = "^2.5.3"
 wemake-python-styleguide = "^0.14.1"  # flake8 plug-in
+
+# Test Suite
+packaging = "^20.4"  # used to test the packaged version
+pytest = "^6.0.1"
+pytest-cov = "^2.10.0"