Restructure test infrastructure for Python SDK

We need to support newer Python versions like 3.8, 3.9 continuously.
But maintaining test for all versions costs us much in point of development and test resources.
So we should reconsider test infrastructure about following points;
- how to prioritize Python versions and tests to be tested frequently, and vice versa
- how to switch easily which versions to have high priority

detail discussion is here:
[https://lists.apache.org/thread.html/rd070afcebff5c967ec3b25d1f7a77db5278992c1508082bf5f636acd%40%3Cdev.beam.apache.org%3E](https://lists.apache.org/thread.html/rd070afcebff5c967ec3b25d1f7a77db5278992c1508082bf5f636acd%40%3Cdev.beam.apache.org%3E)

Imported from Jira [BEAM-9829](https://issues.apache.org/jira/browse/BEAM-9829). Original Jira may contain additional context.
Reported by: yoshiki.obata.