bpo-46242: better error message for extending Enum with members

[sobolevn]

Three major changes:

1. Better error message, which is more informative and is in sync with

   cpython/Lib/enum.py

   Lines 850 to 853 in 549e628

   	raise TypeError(
   	"%s: cannot extend enumeration %r"
   	% (class_name, base.__name__)
   	)

   Another idea is too call _check_for_existing_members on first_enum in _get_mixins_. But, since _get_mixins_ is @staticmethod, I've decided not to use explicit EnumType._check_for_existing_members() or not to convert it to @classmethod
2. For some reason in _create_ wrong type was passed to _get_mixins_, instead of str, Enum was passed
3. I've also covered this feature with a test case

https://bugs.python.org/issue46242

[sobolevn]

Another idea is too call _check_for_existing_members on first_enum in get_mixins. But, since get_mixins is @staticmethod, I've decided not to use explicit EnumType._check_for_existing_members() or not to convert it to @classmethod

Here's a commit with this version: 9e7d02c Just to compare. Maybe I even like it more 🤔

[sobolevn]

Friendly CC @ethanfurman
Maybe you have missed this one? 🙂

[ethanfurman]

Thanks, good fix.