Add py39 test environment to docs

[Clivern]

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[asottile]

instead, I think it would be better to mention the heuristics tox uses to create environments here -- that way this doesn't invite more bitrot / noisy PRs as new versions of python comes out

tox currently understands the following patterns:

• py: the current interpreter tox is using
• pypy: whatever available pypy there is
• jython: whatever available jython there is
• py#: python of major version #
• pypy#: pypy of major version #
• py#%%: cpython version #.%%
• pypy#%%: pypy version #.%%

[Clivern]

I updated to naming pattern @asottile, pls check & feel free to update PR if needed.

[gaborbernat]