Re-introduce the 'stubtest' key for third-party packages (#7351)

Some distributions can't be tested with stubtest for a variety of
reasons or because of bugs in stubtest. This key:

* let's us keep metadata about a distribution in one place,
* prevents us from modifying the scripts because of issues with a
  specific distribution, and
* will trigger tests if only the key is changed.

stubs/SQLAlchemy/METADATA.toml

@@ -3,3 +3,4 @@ extra_description = """\
   The `sqlalchemy-stubs` package is an alternative to this package and also \
   includes a mypy plugin for more precise types.\
 """
+stubtest = false  # https://github.com/python/typeshed/issues/7307