Python PreCommit broken

[Maximilian Michels]

Hi,

The Python PreCommit tests are currently broken. Is anybody looking into 
this?

Example: 
https://builds.apache.org/job/beam_PreCommit_Python_Commit/1308/#showFailuresLink
JIRA: https://issues.apache.org/jira/browse/BEAM-5458

I'm sure this was just an accident. No big deal but let's make sure 
PreCommit passes when merging. A broken PreCommit means that we can't 
merge any other changes with confidence.

Thanks,
Max