ci: run full doctests from python 3.6 to 3.10

[juarezr]

This PR has the objective of enabling doctests on python 3.9 and 3.10

Changes

1. Enabled doctests from python 3.6 to 3.10
2. Except on windows that doctests are enabled from python 3.8 to 3.10

Context

• Before we cant do this because python3.10 is not compatible with nosetests.
• But after #584 was merged @arturponinski cleared this issue.

Checklist

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

• [X] Code
  • [X] 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
  • [x] Github Actions passes (unit tests run under Linux)
  • [x] AppVeyor CI passes (unit tests run under Windows)
  • [x] Unit test coverage has not decreased (see Coveralls)
• [ ] Changes
  • [ ] (Optional) Just a proof of concept
  • [ ] (Optional) Work in progress
  • [x] Ready to review
  • [x] Ready to merge

[coveralls]

Coverage increased (+0.9%) to 91.759% when pulling 7e317f88d31dd33d3dffb3e9dac22b33b222575a on juarezr:py3_10full into bf6873fc0f9ba73c2e7299f4e891d4370d71633d on petl-developers:master.

[juarezr]

@arturponinski,

Can you review this PR again, please?

The main changes are:

• Testing with python from 2.7 up to 3.10 in all OS
• Re-enabling doctests for python3 that we both missed in the pytest migration
• Skipping some library incompatibilities with py3.10
• So now we are on par with nosetests :smile:

[arturponinski]

@juarezr I've made some minor worthy changes, and made a PR to your branch: https://github.com/juarezr/petl/pull/4