ELEX-4647 more unit tests

[dmnapolitano]

Description

Hi! The changes in this PR:

• address pylint complaints, although TODO ones still remain;
• adds a BaseDataHandler abstract class that most DataHandlers are now subclasses of;
• adds some unit tests for VersionedDataHandler although I'm sure there's more scenarios we want to test there though I'm not exactly sure what 🤔
• ensures that all top-level imports of external packages are dependencies in setup.py;

Thanks! 🎉

Jira Ticket

ELEX-4647

Test Steps

tox or the elexmodel command of your choice 🎉