Tox-related updates; drop references to py35 and py36

[fselmo]

What was wrong?

• We have some issues with some tox updates causing the CI docs job to fail.
• We also had some remaining references to python 3.5 and 3.6

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Note: Shanghai was also added to the ForkName enum in this commit. Since that was a simple push with no PR, I am adding the newsfragment reference to this PR with this comment to track it.

How was it fixed?

• Change whitelist_externals -> allowlist_externals
• Remove references to unsupported python versions
• Add comment in PR linking to the Shanghai enum feature commit. Add feature newsfragment for this change.

To-Do

• [x] Clean up commit history
• [x] Add entry to the release notes

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