This PR fixes the issue E: Package 'python3-distutils' has no installation candidate which can be seen in this build log from PR googleapis/python-pubsub#1220.

This PR also updates the default python version to be 3.10 for the docs session to match what we use for docfx.

I tested that this build successfully by running docker build -t test_docs . in the directory synthtool/gcp/templates/python_library/.kokoro/docker/docs/

I then ran the following nox sessions docs and docfx.

Note: The both the docs and docfx session now require Python 3.10.

docker run --rm -it --entrypoint /bin/bash test_docs

Then in the docker image, run the following commands

git clone https://github.com/googleapis/python-pubsub.git
cd python-pubsub
nox -s docs
nox -s docfx