Correct str.maketrans and collections.UserString.maketrans

[AlexWaygood]

Refs #9608.

If the third argument for str.maketrans is not specified, dict[int, int] is returned. If the third argument is specified, it must be str, not None:

>>> str.maketrans('a', 'b')
{97: 98}
>>> str.maketrans('a', 'b', None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: maketrans() argument 3 must be str, not None

This all aligns with the documentation: https://docs.python.org/3/library/stdtypes.html#str.maketrans

At runtime, collections.UserString.maketrans is just a simple alias for str.maketrans: https://github.com/python/cpython/blob/666c0840dcac9941fa41ec619fef8d45cd849a0b/Lib/collections/__init__.py#L1527. We can do the same in the stub, and should do the same in the stub; it will ensure that the two methods remain in sync. (Currently the parameters for UserString.maketrans are incorrectly marked as positional-or-keyword, for example -- they are in fact positional-only.)

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉