docs(python): document adding Python 3.9 support, dropping 3.5 support (#120)

Closes #787

Source-Author: Tres Seaver <tseaver@palladion.com>
Source-Date: Thu Dec 17 16:08:02 2020 -0500
Source-Repo: googleapis/synthtool
Source-Sha: b670a77a454f415d247907908e8ee7943e06d718
Source-Link: https://github.com/googleapis/synthtool/commit/b670a77a454f415d247907908e8ee7943e06d718
2 files changed