Drop RHEL 7 / CentOS 7 support and move to Python 3.8 minimum?

[Andrew Bartlett]

I was reviewing patches recently when it was mentioned at
https://gitlab.com/samba-team/samba/-/merge_requests/3228/diffs?commit_id=0252ca68359a330a92f9dad01d2c2cd037386c6f
that addClassCleanup() would be a cleaner way to work.

We need to reduce our CI time, and one way to do that is to reuse
objects as much as possible, creating read-only objects once per class,
not per test.  

This is much cleaner if with the addCleanup() and addClassCleanup()
methods, so I would like to be able to do that.

To do that we need Python 3.8.  The primary blocker for this is the
CentOS 7 build which uses Python 3.6 from EPEL, and a newer Python is
not in that repo.

Centos 7 is already only working because of an externally maintained
GnuTLS package, not in any distribution similar. 

I propose that we drop the CentOS 7 CI build, also reducing our CI
count, and increase our python requirement to 3.8.

The other interesting feature, while not a major change, is the =
support in f-strings:
https://docs.python.org/3/whatsnew/3.8.html#f-strings-support-for-self-documenting-expressions-and-debugging

What do folks think?  I know CentOS 7 was for a while quite important
due to the changes in the RHEL clone landscape (CentOS stream etc), so
I'm keen to hear views.
Andrew Bartlett
-- 
Andrew Bartlett (he/him)       https://samba.org/~abartlet/Samba Team Member (since 2001) https://samba.orgSamba Team Lead                https://catalyst.net.nz/services/sambaCatalyst.Net Ltd
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