recheck comment about python3.9 compatibility in docs

[MoralCode]

augur/docs/source/getting-started/installation.rst

Line 34 in 2c9a5f1

**Python 3.9 is not yet supported because TensorFlow, which we use in our machine learning workers, does not yet support Python 3.9.**

OK ... wow. This is out of date.. Noting the need to fix it later.

Originally posted by @sgoggins in #3183 (comment)