Update documented python version support; add py3.9 test configuration

[kandersolar]

I noticed a few places in the docs that mentioned older python versions that are no longer supported as of #112 and updated them here. I think the PyPI badge in the readme will automatically update once the changes to setup.py are propagated to pypi.org. I also added a python 3.9 test configuration.

I assume the tests will fail because of the same sphinx issue in #121 so I'm marking this as draft and will merge master in after #121 is complete :)

Side notes:

1. Looks like the docs on gh-pages are still for 1.5.0 and could use a rebuild for 1.5.1
2. I don't think this is a pressing issue (though I have almost no experience w/ CircleCI so maybe I'm wrong), just FYI: the docker images used in pvfactor's CI config are apparently considered "legacy" and these new images should be preferred: https://circleci.com/developer/images/image/cimg/python

[kandersolar]

Okay @campanelli-sunpower @anomam I think this is ready. The 3.7 job is still building pandas from source; I did not investigate why. Python 3.10.0 was released recently, but not all of the core pydata packages have released 3.10 wheels yet (matplotlib, maybe others), so I did not add a 3.10 job here, but probably it should be added at some point before too long.

Thanks!

[campanelli-sunpower]

@kanderso-nrel I updated the documentation to v1.5.1 at https://sunpower.github.io/pvfactors/, and I should have a small PR coming for your review to fix some documentation issues that I discovered along the way, which should go in before we release this. Also, I'd like to fix the docker image issue you spotted, as well as automate documentation builds and deploys on each tag, but I won't hold up the release if that CI work ends up taking more time than I have now.

I should be reviewing this PR shortly.

[campanelli-sunpower]

Checked that 3.9 tests are passing in CI with same count as 3.8 tests. Thanks for the updates @kanderso-nrel.