Python Post Commit 3.6 issue

https://builds.apache.org/job/beam_PostCommit_Python36/469/console

12:04:37 ======================================================================
12:04:37 ERROR: Failure: ModuleNotFoundError (No module named 'dataclasses')
12:04:37 ----------------------------------------------------------------------
12:04:37 Traceback (most recent call last):
12:04:37 File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python36/src/build/gradleenv/-1734967053/lib/python3.6/site-packages/nose/failure.py", line 39, in runTest
12:04:37 raise self.exc_val.with_traceback(self.tb)
12:04:37 File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python36/src/build/gradleenv/-1734967053/lib/python3.6/site-packages/nose/loader.py", line 418, in loadTestsFromName
12:04:37 addr.filename, addr.module)
12:04:37 File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python36/src/build/gradleenv/-1734967053/lib/python3.6/site-packages/nose/importer.py", line 47, in importFromPath
12:04:37 return self.importFromDir(dir_path, fqname)
12:04:37 File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python36/src/build/gradleenv/-1734967053/lib/python3.6/site-packages/nose/importer.py", line 94, in importFromDir
12:04:37 mod = load_module(part_fqname, fh, filename, desc)
12:04:37 File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python36/src/build/gradleenv/-1734967053/lib/python3.6/imp.py", line 235, in load_module
12:04:37 return load_source(name, filename, file)
12:04:37 File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python36/src/build/gradleenv/-1734967053/lib/python3.6/imp.py", line 172, in load_source
12:04:37 module = _load(spec)
12:04:37 File "<frozen importlib._bootstrap>", line 684, in _load
12:04:37 File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
12:04:37 File "<frozen importlib._bootstrap_external>", line 678, in exec_module
12:04:37 File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
12:04:37 File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python36/src/sdks/python/apache_beam/transforms/external_test_py37.py", line 22, in <module>
12:04:37 import dataclasses

I am guessing that https://github.com/apache/beam/pull/9570 will suppress this. However there might be additional changes needed after https://github.com/apache/beam/pull/9098 for python 3.6.

/cc tvalentyn