Figure out min supported Python 3 version

[kennknowles]

We have 3.4.3 installed on Jenkins workers. We could target that as we add support, but in th long run we will need to figure out the supported version story for python 3.

Imported from Jira BEAM-3781. Original Jira may contain additional context.
Reported by: altay.

[Abacn]

.close-issue not_planned