Python 3.5 EOL; when can require 3.6?

[Markus Armbruster]

We require Python 3.5.  It will reach its "end of life" at the end of
September 2020[*].  Any reason not to require 3.6 for 5.2?  qemu-iotests
already does for its Python parts.


[*] https://www.python.org/downloads/release/python-3510/