stubtest and @deprecated

Bug Report

The @warnings.deprecated decorator adds a few dunders to classes it decorates. This causes stubtest to complain about differences between stub and implementation, unless these changes are added manually to the stubs. I think we should special case @deprecated to ignore these differences.

To Reproduce

For example, adding the following test to StubtestUnit:

    @collect_cases
    def test_deprecated(self) -> Iterator[Case]:
        yield Case(
            stub="""
            from warnings import deprecated

            @deprecated("")
            class Foo: ...
            """,
            runtime="""
            from warnings import deprecated

            @deprecated("")
            class Foo:
                pass
            """,
            error=None,
        )

causes the following error when running the tests with Python 3.13:

E           AssertionError: error: test_module.Foo.__deprecated__ is not present in stub
E             Stub: in file test_module.pyi
E             MISSING
E             Runtime:
E             ''
E             
E             error: test_module.Foo.__init_subclass__ is not present in stub
E             Stub: in file test_module.pyi
E             MISSING
E             Runtime: in file /opt/pyenv/versions/3.13.8/lib/python3.13/warnings.py:621
E             def (*args, **kwargs)
E             
E             error: test_module.Foo.__new__ is not present in stub
E             Stub: in file test_module.pyi
E             MISSING
E             Runtime: in file /opt/pyenv/versions/3.13.8/lib/python3.13/warnings.py
E             def (*args, **kwargs)
E             
E             Found 3 errors (checked 1 module)
E             
E           assert {'test_module....Foo.__new__'} == set()
E             
E             Extra items in the left set:
E             'test_module.Foo.__deprecated__'
E             'test_module.Foo.__new__'
E             'test_module.Foo.__init_subclass__'
E             Use -v to get more diff

Interestingly, this is different from what we see in typeshed (python/typeshed#14893):

error: jwcrypto.jwt.JWTMissingKeyID.__init_subclass__ is inconsistent, stub does not have *args parameter "args"
Stub: in file /home/runner/work/typeshed/typeshed/stubs/jwcrypto/jwcrypto/jwt.pyi:139
def (cls: type[builtins.object])
Runtime: in file /opt/hostedtoolcache/Python/3.13.8/x64/lib/python3.13/warnings.py:621
def (*args, **kwargs)

Your Environment

• Mypy version used: master for the test above, 1.18.2 in typeshed CI
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.13

Start with the proposed test in StubtestUnit and reproduce it on Python 3.13. Trace stubtest’s comparison of classes decorated with warnings.deprecated, then make the test pass while ignoring only the decorator-added differences.