Tagging python transform class as experimental breaks tests

As part of BEAM-7715 I tried to add the `@experimental()` annotation in the external.py `External` and generate_sequence transform too. However it broke the tests apparently on a typing issue related to the annotation.

Log:
15:53:42 File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_PVR_Flink_Commit/src/sdks/python/apache_beam/runners/portability/flink_runner_test.py", line 172, in test_external_transforms*15:53:42* expansion_service=get_expansion_service()))}}
15:53:42 File "apache_beam/utils/annotations.py", line 146, in inner*15:53:42* return fnc(args, **kwargs)*
15:53:42 File "apache_beam/io/external/generate_sequence.py", line 56, in _init
__15:53:42 super(GenerateSequence, self).init_()15:53:42 TypeError: super() argument 1 must be type, not functionRef.

Ref.
https://builds.apache.org/job/beam_PreCommit_Python_PVR_Flink_Commit/3794/console