Revert old python support removal

[xrmx]

What does this pull request do?

This reverts 8ac1781 and 76cf5c8. The plan is to release a minor version first so we cannot break backwards compatibility.

Related issues

Refs #2029

[github-actions]

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🔍 Preview links for changed docs

• docs/reference/run-tests-locally.md

[trentm]

Er, though tests for Python 3.6 are failling.

[xrmx]

Ok so mock backport module was there for a reason, with 3.6 and 3.7 stdlib unittest mock we were not able to properly assert kwargs on a Mock instance.

[xrmx]

Mongodb tests fixed here #2391