Deprecate Python 3.7 & 3.8 support

[Repiteo]

Originally part of #4825, this PR simply deprecates Python 3.7 & 3.8. The only other change is removing a check in ActionTests.py, as I realized that was conditional on 3.6 specifically, so it being bumped to 3.7 was a mistake

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]

The deprecated 3.7 failed a test (SCONSFLAGS.py(. Side effect of us being so fussy about exact outputs - the (expected) presence of a deprecation message shouldn't blow it up...

[bdbaddog]

The deprecated 3.7 failed a test (SCONSFLAGS.py(. Side effect of us being so fussy about exact outputs - the (expected) presence of a deprecation message shouldn't blow it up...

Didn't we handle this in the testing framework? (or is my memory failing me on this one?)

[mwichmann]

There's something for the deprecation itself. There's not for an individual test that gets extra output due to using a deprecated Python version. I'm kind of surprised it hit only one test. We should be able to come up with something - it might possibly be related to the change to runtest to add logging.

[bdbaddog]

There's something for the deprecation itself. There's not for an individual test that gets extra output due to using a deprecated Python version. I'm kind of surprised it hit only one test. We should be able to come up with something - it might possibly be related to the change to runtest to add logging.

Looks like the failing part is some additional checks @mwichmann added in 2024, and we've not had any deprecations since then so it didn't get hit til now..
Taking a look at a fix now.