Drop Python 3.6 and 3.7 support

[osandov]

Python 3.6 has been EOL since December 2021, over 3 years ago. Python 3.7 has been EOL since June 2023, about a year and a half. drgn has kept supporting both as a courtesy to enterprise Linux distros, but it's a burden for a few reasons:

1. The ecosystem has largely dropped support for Python 3.6 and 3.7. Mypy and other pre-commit hooks no longer support them, so we already have to skip those for 3.6 and 3.7. (We already depend on a few typing features that don't exist in 3.6 or 3.7 since we can't type check it anyways.)
2. meson-python requires Python >= 3.7, which blocks converting from our setuptools/autotools mess to Meson.
3. GitHub Actions is deprecating Ubuntu 20.04, the last version with Python 3.6 and 3.7 support, in April 2025: The Ubuntu 20.04 Actions runner image will begin deprecation on 2025-02-01 and will be fully unsupported by 2025-04-01 actions/runner-images#11101.
4. (Least importantly) there are a few language/standard library/C API features from 3.8+ that would be nice to use; search for comments mentioning "3.6" or "3.7" in the codebase.

Converting to Meson is the pain point I feel the most right now, so I would really like to drop Python 3.6 support. I'm also not keen to do a bunch of work to address the GitHub Actions image deprecation just to keep testing 3.6 and 3.7, and without CI I'm doubtful that we'll be able to keep the support working.

That said, I don't want to do this lightly. Python 3.6 is used by RHEL 7 and 8. Python 3.7 is used by Debian Buster (oldoldstable at the time of writing).

The current plan is to drop support for both Python 3.6 and 3.7 in 2 releases (drgn 0.0.32). Please comment here if you would like me to reconsider.

[brenns10]

The Enterprise Linux person in me is sad to see it go, but the drgn developer in me says good riddance :)

Thanks for the long period of extra support for 3.6! Making 0.0.31 the terminal release for it will be great since it will contain some really exciting features.