Posting this in case someone hits the same issue.
Since python 3.12, UserDict defines it's own get(), which check key in self first. Because __contains__ does not call __missing__, a missing key returns the default instead.

CPython 3.12+ UserDict:

    # Modify __contains__ and get() to work like dict
    # does when __missing__ is present.
    def __contains__(self, key):
        return key in self.data

    def get(self, key, default=None):
        if key in self:
            return self[key]
        return default

So in 03-dict-set/missing.py, this doctest fails on Python ≥ 3.12:

    >>> ud = UserDictSub(A = 'letter A')
    >>> ud['a']  # ✅
    'letter A'
    >>> ud.get('a', '')  # ❌
    'letter A'