python: export pytest.Metafunc for typing purposes

The type cannot be constructed directly, but is exported for use in type
annotations, since it is reachable through existing public API.

testing/python/metafunc.py

@@ -47,7 +47,7 @@ class TestMetafunc:
         names = getfuncargnames(func)
         fixtureinfo: Any = FuncFixtureInfoMock(names)
         definition: Any = DefinitionMock._create(func, "mock::nodeid")
-        return python.Metafunc(definition, fixtureinfo, config)
+        return python.Metafunc(definition, fixtureinfo, config, _ispytest=True)
 
     def test_no_funcargs(self) -> None:
         def function():