Add __version__ identifier

- `lalib.__version__` is dynamically assigned
- the format is "x.y.z[.dev0|aN|bN|rcN|.postM]"
  where x, y, z, M, and N are non-negative integers
  + x, y, and z model the "major", "minor", and "patch"
    parts of semantic versioning
    (See: https://semver.org/#semantic-versioning-200)
  + M is a single digit and N either 1 or 2
  => This complies with (a strict subset of) PEP440
- add unit tests for the `__version__` identifier

README.md

@@ -119,3 +119,14 @@ Whereas a rebase makes a simple fast-forward merge possible,
     all merges are made with explicit and *empty* merge commits.
 This ensures that past branches remain visible in the logs,
     for example, with `git log --graph`.
+
+
+#### Versioning
+
+The version identifiers adhere to a subset of the rules in
+    [PEP440](https://peps.python.org/pep-0440/) and
+    follow [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+So, releases to [PyPI](https://pypi.org/)
+    come in the popular `major.minor.patch` format.
+The specific rules for this project are explained
+    [here](https://github.com/webartifex/lalib/blob/main/tests/test_version.py).