CI/DOC: temporary pin environment to python 3.8
The doc build is failing on master (`/home/runner/work/pandas/pandas/doc/source/reference/api/pandas.Series.dt.rst: WARNING: document isn't included in any toctree`), and the build that starts failing seems to have switched from python 3.8 to 3.9 (I don't see any other relevant change in the environment). I suppose the default python changed at the conda(-forge) side.