py3.10: start testing petl compatibility

[juarezr]

This PR has the objective of testing compatibility with python 3.10.

Changes

1. Added a new matrix option to Github Action
2. Fixed a bug in...
3. Changed the behaviour of...
4. Improved the docs about...

Checklist

Checklist for pull requests including new code and/or changes to existing code...

• [ ] Code
  • [ ] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [ ] Docstrings include notes for any changes to API or behaviour
  • [ ] All changes documented in docs/changes.rst
• [ ] Testing
  • [ ] ˜(Optional) Tested local against remote servers˜
  • [ ] Travis CI passes (unit tests run under Linux)
  • [ ] AppVeyor CI passes (unit tests run under Windows)
  • [ ] Unit test coverage has not decreased (see Coveralls)
• [ ] Changes
  • [ ] ˜(Optional) Just a proof of concept˜
  • [X] (Optional) Work in progress
  • [ ] Ready to review
  • [ ] Ready to merge

[coveralls]

Coverage remained the same at 90.871% when pulling a91b568115369ce091d9d3d9c24477ae24662aac on juarezr:py3_10 into a8e8e3f12a1404b6e9bc226df44cf8b11214b400 on petl-developers:master.

[juarezr]

After #584 CI is passing the tests with python3.10 for:

• Linux (Ubuntu-latest)
• Windows
• macOS
• Simple Mode: no doctest yet