Support Python3.10/3.11, documentation fixes, and general codebase cleanup

[Zeitsperre]

Overview

This PR does a bit of cleaning in the code base:

• Many unused imports have been removed.
• Many Markdown and reST formatting issues (outside the docs) have been corrected.
• Some docstrings for functions have been cleaned up for typos and better rendering in the documentation.
• Some URL targets that pointed to insecure addresses (http://) have been promoted to secured ones (https://).
• References to the master branch have been changed to main.
• Updated Installation instructions to reflect modern approaches.
• Added builds on GitHub CI for Python3.10 and Python3.11.
• Resolved a DeprecationWarning emitted by sqlalchemy (backwards-compatible).
• Ran isort over the code base to organize imports.

Related Issue / Discussion

Closes #673
Closes #637
Closes #590

Addresses some comments in the following issues:
#669
#336

Additional Information

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

coverage: 81.332% (-0.02%) from 81.35% when pulling a6ebbea on Zeitsperre:housekeeping into c56aba6 on geopython:main.

[huard]

Looks good, Nice clean-up.

[Zeitsperre]

I'm happy with this for now. There are more than a few modules that looks to be obsolete, so I'll stop here.

@cehbrecht I'll leave the final review, merge, and release to you.

If you'd like me to add more to the CI/pre-commit side of things, let me know and I'll open another PR for those.

[cehbrecht]

@Zeitsperre looks all good 👍 Thanks for the clean-up 🙂

[cehbrecht]

If you'd like me to add more to the CI/pre-commit side of things, let me know and I'll open another PR for those.

@Zeitsperre sounds good to add the CI/pre-commit things 👍