Bump Python requirement to at least 3.8

[PGijsbers]

And then follow NEP29 or other deprecation schedule (python release schedule).

The Python release schedule states:

• annual release
• during the first eighteen months (1½ year) [after initial release of the 3.X version], it receives bugfix updates and full releases (sources and installers for Windows and macOS) are made approximately every other month.

I'm leaning towards supporting only the Python releases for which full releases are available (i.e., up to 1½ years old).
Perhaps in time we can also add a second line of support for bug fixes on older releases, but right now it seems like the burden will outweigh any benefits.

Note that supporting only Python 3.X+ does not mean it won't work on 3.(X-1)-.
It just means that we don't test on those versions, and don't do version specific changes.
Compatibility only breaks when using specific new language features (e.g., syntax, or the built-ins of 3.(X-1)- are deprecated).

[PGijsbers]

Closed #158