Timeout in _verify_runner_api_compatible

[kennknowles]

Some Dataflow tests sometimes are getting stuck in _verify_runner_api_compatible

https://builds.apache.org/view/Beam/job/beam_PostCommit_Python_Verify/1774/console
https://builds.apache.org/view/Beam/job/beam_PostCommit_Python_Verify/1769/console

Sample stack:
File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify/sdks/python/apache_beam/transforms/sideinputs_test.py", line 179, in test_iterable_side_input
pipeline.run()
File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify/sdks/python/apache_beam/test_pipeline.py", line 91, in run
result = super(TestPipeline, self).run()
File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify/sdks/python/apache_beam/pipeline.py", line 158, in run
if test_runner_api and self._verify_runner_api_compatible():
File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify/sdks/python/apache_beam/pipeline.py", line 326, in _verify_runner_api_compatible
self.visit(Visitor())
File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify/sdks/python/apache_beam/pipeline.py", line 192, in visit
self._root_transform().visit(visitor, self, visited)
File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify/sdks/python/apache_beam/pipeline.py", line 482, in visit
part.visit(visitor, pipeline, visited)
File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify/sdks/python/apache_beam/pipeline.py", line 485, in visit
visitor.visit_transform(self)
File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify/sdks/python/apache_beam/pipeline.py", line 318, in visit_transform
pickler.loads(pickler.dumps(transform_node.transform))
File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify/sdks/python/apache_beam/internal/pickler.py", line 203, in loads
c = base64.b64decode(encoded)
File "/usr/lib/python2.7/base64.py", line 73, in b64decode
return binascii.a2b_base64(s)
File "/home/jenkins/jenkins-slave/workspace/beam_PostCommit_Python_Verify/sdks/python/nose-1.3.7-py2.7.egg/nose/plugins/multiprocess.py", line 276, in signalhandler
raise TimedOutException()

cc: [~vikasrk]

Imported from Jira BEAM-1916. Original Jira may contain additional context.
Reported by: altay.