Consider eliminating propka/version.py

[speleo3]

Quoting Nathan (9826351 May 2020):

TODO - this module unnecessarily confuses the code. Can we eliminate it?

propka/version.py provides an abstract Version class and several subclass implementations. The only implementation that we currently use is VersionA, which gets selected from propka.cfg. It's unclear whether the other implementations are even working and there is neither documentation nor example code for how and why to use them.

API doc: https://propka.readthedocs.io/en/latest/api/propka.version.html

See also #179