mock.seal has infinite recursion with mutually recursive class references

Bug report

This code seems to cause infinite recursion in unittest.mock.seal:

from unittest import mock
class Foo:
  pass
class Bar:
  foo = Foo
Foo.bar = Bar
foo = mock.create_autospec(Foo)
mock.seal(foo)

$ python3 foo.py 
Traceback (most recent call last):
  File "/tmp/tmp.yKOznvlUIG/foo.py", line 8, in <module>
    mock.seal(foo)
  File "/usr/lib/python3.9/unittest/mock.py", line 2874, in seal
    seal(m)
  File "/usr/lib/python3.9/unittest/mock.py", line 2874, in seal
    seal(m)
  File "/usr/lib/python3.9/unittest/mock.py", line 2874, in seal
    seal(m)
  [Previous line repeated 973 more times]
  File "/usr/lib/python3.9/unittest/mock.py", line 2866, in seal
    m = getattr(mock, attr)
  File "/usr/lib/python3.9/unittest/mock.py", line 655, in __getattr__
    result = create_autospec(
  File "/usr/lib/python3.9/unittest/mock.py", line 2627, in create_autospec
    mock = Klass(parent=_parent, _new_parent=_parent, _new_name=_new_name,
  File "/usr/lib/python3.9/unittest/mock.py", line 2034, in __init__
    _safe_super(MagicMixin, self).__init__(*args, **kw)
  File "/usr/lib/python3.9/unittest/mock.py", line 1074, in __init__
    _safe_super(CallableMixin, self).__init__(
  File "/usr/lib/python3.9/unittest/mock.py", line 437, in __init__
    self._mock_add_spec(spec, spec_set, _spec_as_instance, _eat_self)
  File "/usr/lib/python3.9/unittest/mock.py", line 500, in _mock_add_spec
    res = _get_signature_object(spec,
  File "/usr/lib/python3.9/unittest/mock.py", line 107, in _get_signature_object
    return func, inspect.signature(sig_func)
  File "/usr/lib/python3.9/inspect.py", line 3113, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped)
  File "/usr/lib/python3.9/inspect.py", line 2862, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
  File "/usr/lib/python3.9/inspect.py", line 2333, in _signature_from_callable
    wrapped_sig = _get_signature_of(obj.func)
  File "/usr/lib/python3.9/inspect.py", line 2329, in _signature_from_callable
    return _signature_from_builtin(sigcls, obj,
  File "/usr/lib/python3.9/inspect.py", line 2149, in _signature_from_builtin
    return _signature_fromstr(cls, func, s, skip_bound_arg)
  File "/usr/lib/python3.9/inspect.py", line 2009, in _signature_fromstr
    _signature_strip_non_python_syntax(s)
  File "/usr/lib/python3.9/inspect.py", line 1962, in _signature_strip_non_python_syntax
    for t in token_stream:
  File "/usr/lib/python3.9/tokenize.py", line 525, in _tokenize
    pseudomatch = _compile(PseudoToken).match(line, pos)
  File "/usr/lib/python3.9/tokenize.py", line 99, in _compile
    return re.compile(expr, re.UNICODE)
  File "/usr/lib/python3.9/re.py", line 252, in compile
    return _compile(pattern, flags)
  File "/usr/lib/python3.9/re.py", line 292, in _compile
    flags = flags.value
  File "/usr/lib/python3.9/types.py", line 178, in __get__
    return self.fget(instance)
RecursionError: maximum recursion depth exceeded

Your environment

• CPython versions tested on: 3.9.9
• Operating system and architecture: Debian testing, amd64

从 unittest/mock.py 中的 unittest.mock.seal 和 create_autospec 入手，使用提供的 Foo/Bar 复现代码跟踪递归路径。当 mock.seal(foo) 运行时，互相递归的类引用不再导致无限递归或 RecursionError，即表示完成。