One painful one that is still reverberating a bit in some areas is the renaming of "SafeConfigParser" to just "ConfigParser" in the standard library (in 3.12). This caused a whole lot of breaking in some areas because versioneer (a package for determining a package version from git tags) used it (in code that was placed inside your package, and so couldn't be solved by just upgrading versioneer).

Also, I'm starting to get warning about something in tarfile that I will need to track down: https://peps.python.org/pep-0706/

There's many, but here's just one.

    Python 3.7.9 (default, Aug 23 2020, 00:57:53)
    [Clang 10.0.1 ] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import cgi
    >>>

    Python 3.13.0 (main, Oct  8 2024, 01:04:00) [Clang 18.1.8 ] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import cgi
    Traceback (most recent call last):
      File "<python-input-0>", line 1, in <module>
        import cgi
    ModuleNotFoundError: No module named 'cgi'

I think distutils is a good example of that (though imo it's a justified break, but still)