Python 3.9+ documentation

[jamesw966]

lambci images stopped at Python 3.8 and won't have further development. For Python 3.9 and 3.10, I've tested ones at mlupin/docker-lambda (e.g. tag "python3.9-build") and they build fine. I don't have time to do a comprehensive doc update PR but if anyone is looking for Python 3.9+ support then try those out.