Add support for Python 3.10

[hugovk]

Thanks for submitting a PR, your contribution is really appreciated!

Here's a quick checklist that should be present in PRs:

• Make sure to include reasonable tests for your change if necessary

• We use towncrier for changelog management, so please add a news file into the changelog folder following these guidelines:

  • Name it $issue_id.$type for example 588.bugfix;

  • If you don't have an issue_id change it to the PR id after creating it

  • Ensure type is one of removal, feature, bugfix, vendor, doc or trivial

  • Make sure to use full sentences with correct case and punctuation, for example:

    Fix issue with non-ascii contents in doctest text files.
    

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Python 3.10.0 final is due for release in October:

• https://www.python.org/dev/peps/pep-0619/

The final release candidate is now out and the Python release team has issued a call to action for community members:

We strongly encourage maintainers of third-party Python projects to prepare their projects for 3.10 compatibilities during this phase. As always, report any issues to the Python bug tracker.

https://discuss.python.org/t/python-3-10-0rc2-is-now-available/10496?u=hugovk

So let's also test and add support for 3.10.

[nicoddemus]

Thanks!