Revert "Merge pull request #633 from cherrypy/debt/drop-py37"

[webknjaz]

This reverts commit f82aee613ca4f7c0ae80452fedc95b7a89b207d1, reversing changes made to 2b3b3eb4df3868ac59a783a54e83eb55db57b241.

❓ What kind of change does this PR introduce?

• [ ] 🐞 bug fix
• [x] 🐣 feature
• [ ] 📋 docs update
• [x] 📋 tests/coverage improvement
• [ ] 📋 refactoring
• [x] 💥 other

📋 What is the related issue number (starting with #)

N/A

❓ What is the current behavior? (You can also link to an open issue here)

Python 3.6 and 3.7 are not tested in CI and are marked as unsupported in the metadata.

❓ What is the new behavior (if this is a feature change)?

Dropping Python 3.6/3.7 is reverted.

📋 Other information:

This is a check for whether the project can still run in the Python 3.6 runtime as needed by one of the downstream projects (Ceph). It is not intended to be merged into main but may end up in a maintenance branch at some point, if I decide to keep a branch with backports for the 10.x stream.

📋 Contribution checklist:

(If you're a first-timer, check out this guide on making great pull requests)

• [x] I wrote descriptive pull request text above
• [x] I think the code is well written
• [x] I wrote good commit messages
• [x] I have [squashed related commits together][related squash] after the changes have been approved
• [x] Unit tests for the changes exist
• [x] Integration tests for the changes exist (if applicable)
• [x] I used the same coding conventions as the rest of the project
• [ ] The new code doesn't generate linter offenses
• [ ] Documentation reflects the changes
• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences

[related squash]: https://github.com/todotxt/todo.txt-android/wiki/Squash-All-Commits-Related-to-a-Single-Issue-into-a-Single-Commit

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This change is 

[webknjaz]

This is now a part of maint/10.0.x and v10.0.1. maint/10.0.x is going to be merged back into main with a few commits re-reverted.