Expand test matrix to include python 3.9

[kandersolar]

Python 3.9.0 was released recently. It would be good to verify that RdTools has no issues running on it and add 3.9 to our CI test matrix. CIs are starting to support 3.9:

• https://travis-ci.community/t/python-3-9-0-build/10091/13
• Add Python 3.9 support actions/setup-python#148 (comment)

However, our dependencies haven't all released wheels for 3.9 yet (see the numpy news post on Sep 14 2020: https://numpy.org/news/)

• https://pypi.org/project/numpy/#files
• https://pypi.org/project/pandas/#files
• https://pypi.org/project/statsmodels/#files and Installation Failed on Windows 10 with python 3.9 statsmodels/statsmodels#7107 (comment)
• https://pypi.org/project/matplotlib/#files
• https://pypi.org/project/scipy/#files and Failed To Install SciPy On Windows with Python 3.9 scipy/scipy#12923
• https://pypi.org/project/h5py/#files

Cristoph Gohlke has released windows 3.9 wheels, interestingly: https://www.lfd.uci.edu/~gohlke/pythonlibs/

I'm -1 on trying to support 3.9 until the core pydata packages release wheels for it, otherwise we'll be spending an eternity compiling them from source. I'm guessing not many people will want to use RdTools on 3.9 until those packages release wheels anyway. When those wheels get rolled out we can take another look at officially supporting 3.9.

[kandersolar]

I cobbled together a windows py3.9.0 environment using Cristoph Gohlke's wheels with the exception of h5py. All tests passed (except the clear-sky temperature file, of course). ✔️