Fix Enum classes and their inheritance

[roaffix]

• Remove the check for enum import.
  Motivation: check for enum's import error is unnecessary on modern python versions because its built-in library is available with a basic python build version. It is pointless to cover very-very-very situational python builds that could not contain enum in it.
• Remove the C++ style formatting for constants declaration.
  Motivation: FYI, @syurkevi : https://www.python.org/dev/peps/pep-0008/#other-recommendations. Especially example before link that is under "More than one space around an assignment (or other) operator to align it with another" and highlighted "never use more than one space" 🖕 😎
• Remove wrap for Enum constants with _Enum_Type function.
  Motivation: There is no point to use such a method like _Enum_Type that returns the arg as it is passed into it.
• to_str is moved to the "library" module to avoid circular import with the "util" module.
  Note: This change does not break the public API because it is still declared in __init__.