Fix stubtest for sqlalchemy.testing.plugin.pytestplugin (#9533)

stubs/SQLAlchemy/@tests/requirements-stubtest.txt

@@ -0,0 +1 @@
+pytest

stubs/SQLAlchemy/@tests/stubtest_allowlist.txt

@@ -59,10 +59,7 @@ sqlalchemy.testing.provision.stop_test_class_outside_fixtures
 sqlalchemy.testing.provision.temp_table_keyword_args
 sqlalchemy.testing.provision.update_db_opts
 
-# requires other packages
-sqlalchemy.testing.plugin.pytestplugin
-
-# can't be imported
+# KeyError/AttributeError on import due to dynamic initialization from a different module
 sqlalchemy.testing.fixtures
 sqlalchemy.testing.pickleable
 sqlalchemy.testing.plugin.bootstrap