Drop support for python 3.6, transition to PEP-621 pyproject.toml, and misc project updates

[Secrus]

Decided to put a few minutes into updating the project.

• dropped Python 3.6, since it's no longer supported,
• added support for Python 3.9 and 3.10,
• added common virtualenv directory names to .gitignore,
• updated tools URLs and versions in .pre-commit-config.yaml,
• migrated project metadata to PEP-621 spec in pyproject.toml,
• moved .coveragerc data to pyproject.toml (and removed unnecessary entries)

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[Secrus]

@consideRatio I have refreshed the PR (Python 3.11 and updated tools versions). Please re-review.

[Secrus]

I will look at fixing this within the next few days.

[consideRatio]

Observed failures

nox > Command flit install -s --deps=production --extras test failed with exit code 1:
Unexpected names under [project]: home-page         W-flit_core.config
license field should be <class 'dict'>, not <class 'str'>

[consideRatio]

@Secrus I became too curious and ended up resolving this. Can you review my added commits, and if they look good to you I figure we go for a merge!

0da788a - Supplemental fixes to pyproject.toml 87cf689 - ci: update versions of github workflow's actions 10de017 - pre-commit: fix config of black

[Secrus]

@consideRatio LGTM, thanks!

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