Remove support for end-of-life Python versions

[waldyrious]

This is a re-sumbission of #194, which was closed due to the author deleting the fork.

The following version of Python are EOL. They are no longer receiving bug fixes including for security issues. The following table at the following link shows branch status with dates: https://devguide.python.org/devcycle/#end-of-life-branches

By removing EOL Pythons, the code can take advantage of new features and syntax. Such straightforward fixes have been applied using the tool pyupgrade: https://github.com/asottile/pyupgrade

Adopting new syntax will also allow for additional future improvements such as adding type annotations.

Using pypinfo, we can see that that there are no Python 2 users through PyPI:

python_version	percent	download_count
3.10	30.43%	7
3.8	26.09%	6
3.6	21.74%	5
3.9	17.39%	4
3.7	4.35%	1
Total		23