Fix AttributeError when comparing non-Erlang objects to Erlang object…

[nickeldan]

…s. Move common class methods into base class.

[nickeldan]

The check failure may be due to this issue in setuptools.

[okeuday]

@nickeldan Python inheritance is avoided to ensure errors are caught in an explicit way during runtime, despite having a common interface (if it was a programming language with static typing and compile-time type checking, the situation would be different, mypy exists for newer Python 3 versions but is unable to catch every possible problem as an external static analysis tool). There isn't a good backwards compatible (Python 2/3) way to do a pure virtual method (it would make the source code worse if it was possible). For these reasons it is best to not pursue these changes.