Minimally fix CI

[dhdaines]

Pull request

Python versions in CI were very out of date - 3.6 and 3.7 are now end-of-life and no longer supported. See https://devguide.python.org/versions/

But beyond that a lot of things passed CI only because of specific older versions of tooling (black, mypy, pip among other things). These problems should be fixed for real but this will at least get CI working again. I will separate PRs for these problems, which are partly formatting but also some type-checking issues (there are quite a lot of type-checking issues) and of course the issue of -VERSION- as the default version string (really there should be a way to get to semantic versioning...)

How Has This Been Tested?

It wasn't tested when first pushed because there isn't any easy way to test GitHub CI recipes 😡 But now it seems to work :)

Checklist

• [ x ] I have read CONTRIBUTING.md.
• [ x ] I have added a concise human-readable description of the change to CHANGELOG.md.
• [ x ] I have tested that this fix is effective or that this feature works.
• [ x ] I have added docstrings to newly created methods and classes. (not applicable)
• [ x ] I have updated the README.md and the readthedocs documentation. Or verified that this is not necessary. (not applicable)

[pietermarsman]

@dhdaines Thanks for this work! This is a great start to do some more work on pdfminer.six in the nearby future.