Test regression on Python 3.12: AttributeError: type object 'Foo' has no attribute '_is_protocol' #199

$ tox -e py312
.pkg: install_requires> python -I -m pip install 'flit_core<4,>=3.4'
.pkg: _optional_hooks> python /usr/lib/python3.11/site-packages/pyproject_api/_backend.py True flit_core.buildapi
.pkg: get_requires_for_build_sdist> python /usr/lib/python3.11/site-packages/pyproject_api/_backend.py True flit_core.buildapi
.pkg: build_sdist> python /usr/lib/python3.11/site-packages/pyproject_api/_backend.py True flit_core.buildapi
py312: install_package> python -I -m pip install --force-reinstall --no-deps /tmp/typing_extensions/.tox/.tmp/package/1/typing_extensions-4.6.2.tar.gz
py312: commands[0] /tmp/typing_extensions/src> python -m unittest discover
.........................................................................................................................................s..s..s........................................................................................E.................................................................................................s.........
======================================================================
ERROR: test_protocols_isinstance_generic_classes (test_typing_extensions.ProtocolTests.test_protocols_isinstance_generic_classes)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/typing_extensions/src/test_typing_extensions.py", line 2287, in test_protocols_isinstance_generic_classes
    self.assertIsInstance(foo, HasX)
  File "/usr/lib/python3.12/unittest/case.py", line 1292, in assertIsInstance
    if not isinstance(obj, cls):
           ^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/typing.py", line 1815, in __instancecheck__
    if super().__instancecheck__(instance):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen abc>", line 119, in __instancecheck__
  File "/usr/lib/python3.12/typing.py", line 1799, in __subclasscheck__
    return super().__subclasscheck__(other)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen abc>", line 123, in __subclasscheck__
  File "/usr/lib/python3.12/typing.py", line 1902, in _proto_hook
    issubclass(other, Generic) and other._is_protocol):
                                   ^^^^^^^^^^^^^^^^^^
AttributeError: type object 'Foo' has no attribute '_is_protocol'

----------------------------------------------------------------------
Ran 340 tests in 0.149s

FAILED (errors=1, skipped=4)
py312: exit 1 (0.38 seconds) /tmp/typing_extensions/src> python -m unittest discover pid=116427
.pkg: _exit> python /usr/lib/python3.11/site-packages/pyproject_api/_backend.py True flit_core.buildapi
  py312: FAIL code 1 (7.63=setup[7.26]+cmd[0.38] seconds)
  evaluation failed :( (7.85 seconds)