Python postcommit 2.7 tests are failing since we try to run a test with Python 3.6+ syntax on Python 2.

Root cause: https://github.com/apache/beam/pull/9098

20:12:14 Traceback (most recent call last):
20:12:14 File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python2_PR/src/build/gradleenv/-194514014/local/lib/python2.7/site-packages/nose/loader.py", line 418, in loadTestsFromName
20:12:14 addr.filename, addr.module)
20:12:14 File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python2_PR/src/build/gradleenv/-194514014/local/lib/python2.7/site-packages/nose/importer.py", line 47, in importFromPath
20:12:14 return self.importFromDir(dir_path, fqname)
20:12:14 File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python2_PR/src/build/gradleenv/-194514014/local/lib/python2.7/site-packages/nose/importer.py", line 94, in importFromDir
20:12:14 mod = load_module(part_fqname, fh, filename, desc)
20:12:14 SyntaxError: invalid syntax (external_test_py37.py, line 46)