Fix UserDict doctest for Python 3.12 and later - #74

Fixes #73.

UserDict.get() changed in Python 3.12, so the example's expected result now depends on the Python version. This adds a short explanation and checks the matching result in the doctest. All imports, classes and methods outside the module docstring are unchanged.

Checked the module's 22 doctest examples with the standard-library doctest runner on Python 3.12.0 and 3.14.4: all pass. The original example has one failure out of 20 on 3.14.4, at ud.get('a', '').

Python 3.11 wasn't run locally. Its older behavior was checked against the CPython 3.11 implementation and Mapping.get, alongside the 3.12 implementation. This doesn't change the separate missing-key recursion behavior.

Thanks for flagging the version difference.