Drop support for Python 3.5

Supporting Python 3.5.1, 3.5.2 requires nontrivial amount of work in type inference machinery, but may provide little benefit. Most recently, this surfaced in [1].

To require Python 3.5.3 and higher we need to migrate Jenkins infrastructure to Python 3.5.3 (see [2]), and update python_requires section in setup.py once we are ready to drop support.

https://github.com/apache/beam/pull/10821

https://issues.apache.org/jira/browse/BEAM-8152

cc: udim chadrik