Python PostCommit Tests are Flaky

I will use this as an umbrella bug, create sub task and assign to folks working on Python in general. I will try to do a round-robin assignment.

The goal is to improve stability of Py2, 3.5, 3.6, 3.7 postcommits to be working \>80% of the time [1].

/cc (you might get issues assigned) [~pabloem] [~udim] [~chamikara] [~tvalentyn]

[1] http://104.154.241.245/d/McTAiu0ik/stability-critical-jobs-status?orgId=1

Imported from Jira [BEAM-8193](https://issues.apache.org/jira/browse/BEAM-8193). Original Jira may contain additional context.
Reported by: altay.