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