ci: workaround for actions/setup-python#672

[juarezr]

This PR has the objective of fixing actions/setup-python#672.

Changes

1. python 2.7 not available on GitHub latest runners
2. python 3.6 not available on GitHub ubuntu-latest runner

Checklist

Use this checklist to ensure the quality of pull requests that include new code and/or make changes to existing code.

• [ ] Source Code guidelines:
  • [ ] 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 behavior
  • [ ] All changes are documented in docs/changes.rst
• [ ] Versioning and history tracking guidelines:
  • [ ] Using atomic commits whenever possible
  • [ ] Commits are reversible whenever possible
  • [ ] There are no incomplete changes in the pull request
  • [ ] There is no accidental garbage added to the source code
• [ ] Testing guidelines:
  • [ ] Tested locally using tox / pytest
  • [ ] Rebased to master branch and tested before sending the PR
  • [ ] Automated testing passes (see CI)
  • [ ] Unit test coverage has not decreased (see Coveralls)
• [ ] State of these changes is:
  • [ ] Just a proof of concept
  • [ ] Work in progress / Further changes needed
  • [ ] Ready to review
  • [ ] Ready to merge