DOC/CI: fix doc build with Python 3.9
The doc build was failing on master (`/home/runner/work/pandas/pandas/doc/source/reference/api/pandas.Series.dt.rst: WARNING: document isn't included in any toctree`), which started when the environment switched to Python 3.9.

It is now pinned to Python 3.8 (https://github.com/pandas-dev/pandas/pull/41092), but we should fix the warning for Python 3.9, and unpin the environment.

cc @pandas-dev/pandas-docs 