docs: use a newer Python on readthedocs
docs: use a newer Python on readthedocs

They do not seem [1] to support 3.9 yet, so let's stick with the 3.8
version for now.

[1] https://docs.readthedocs.io/en/stable/config-file/v1.html#python-version

Change-Id: I581bde3057a2a33b1ac419cdca3d110d196bdcfe

Telecominfraproject/gnpy