Bump Python version support

[fabianegli]

Currently, tools is built for Python versions 3.6, 3.7 and 3.8. 3.6 is deprecated and does not receive even security updates. This PR drops support for Python 3.6 and adds support for Python 3.9 and 3.10, which results in package target versions for Python 3.7, 3.8, 3.9 and 3.10.

The changes affect the package itself and the GitHub Action linting in the workflow template.

Closes #1621.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #1680 (3fa233f) into dev (ed355ed) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1680   +/-   ##
=======================================
  Coverage   68.00%   68.00%           
=======================================
  Files          56       56           
  Lines        6589     6589           
=======================================
  Hits         4481     4481           
  Misses       2108     2108           

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[apeltzer]

Given that most projects nowadays start going towards 3.9+ already, I think this is a good idea. Shouldn't harm at least :-)