Python 3.6 minimum (drop Ubuntu 16.06 and Debian 9)

Marked as WIP while waiting on discussion on samba-technical.

Set our minimum python version at Python 3.6 for Samba 4.13?

Python 3.6 has useful new features like PEP 498: Formatted string literals

https://docs.python.org/3/whatsnew/3.6.html#whatsnew36-pep498

More particularly, recent tests showed that the only two platforms without it are Ubuntu 16.04 (Xenial) and Debian 9 (Streach).

Debian 10 has been released for 6 months now, and will have been out for more than a year by the time we release Samba 4.13, and likewise there will be a new Ubuntu LTS 20.04.

This came up because a small helper script failed to parse when we extended the samba-o3 tests (which run on each host) to include the local "none" tests !938 (closed)

I realise it would be 'simpler' to just change that script, but I wanted to raise the broader question in the hope that we could steadily increase that minimum version.