Update Python versions

[takis]

I've removed references to Python 3.6 and 3.7 (because of EOL) and increased the minimal Python version to 3.8.
The get method in importlib.metadata.entry_points() was removed in Python 3.12, so I used the select method instead. The select method was added in Python 3.10, so, to make mypy happy, I used the recommended:

if sys.version_info >= (3, 10):

[SmileyChris]

Thanks for the fixes here. I'm going to close in anticipation of #459 getting merged.