Remove Python 3.7 & 3.8 support

[Repiteo]

Originally part of #4825 and building off of #4826, this PR aims to integrate the changes made possible by dropping Python 3.7 & 3.8 support. Marking as a draft until 3.7 & 3.8 are properly designated as "deprecated".

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation

[mwichmann]

Need another way to expressthis - we don't want to turn it on processing into "the release that dropped 3.7" when it should be "the release prior to the one that dropped 3.7".

[bdbaddog]

Understood, but I think in this case our intent is next release 4.10.2 with above message, then following release 4.11.0 and obsolete.. so It'll work for now? LAST_37_RELEASE ? LAST_38_RELEASE ?

[mwichmann]

I think we just have to keep track of that one and fix it manually as appropriate. I'm not saying we need to invent a new mechanism for something that applies to only one feature: a comment about a removed Python. Also, I think we haven't decided whether to "flip the switch" (removal) in the next release or not. Whatever the next release is shouldn't be 4.10.2 anyway, we're beyond "minor bugfix release" at this point.

[Repiteo]

Gonna not even bother with trying to update the documentation/changelog at this time; it gave a ton of conflicts when trying a standard merge commit and just made a ton of noise that would need to be micro-managed. I'll worry about those files once their NEXT_RELEASE is resolved