Add support for Python 3.10

[hugovk]

What do these changes do?

Test on and declare support for Python 3.10.

Are there changes in behavior for the user?

No.

Related issue number

No.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

^ There is no CHANGES folder, is this a global checklist from the org?

[codecov]

Codecov Report

Merging #248 (ce9a1be) into master (beb6f26) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #248   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          107       107           
  Branches        16        16           
=========================================
  Hits           107       107           

Flag	Coverage Δ
unit	98.13% <ø> (ø)

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