add python 3.9 CI configurations

[wholmgren]

• Closes add python 3.9 to ci configuration #1102
• I am familiar with the contributing guidelines
• [ ] Tests added
• [ ] Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• [ ] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels and Milestone are assigned to the Pull Request and linked Issue.

All of the base functionality works. Optional functionality that does not work:

• The forecast.py code needs netcdf4. As of now, netcdf4 needs a different build of numpy than would otherwise be pulled by the environment solver. This build of numpy spits out an ImportError on my mac.
• Numba is not yet compatible with 3.9.
• pysam installation issue on windows. Maybe related to Pip install causing "SAMNIDIR" error NREL/pysam#56. The easiest way to deal with this is to comment out the pysam line in the environment file and thus skip it on all platforms. That's my preference.

[kandersolar]

Guessing the installation failure is related to NREL/pysam#68

[cwhanse]

The easiest way to deal with this is to comment out the pysam line in the environment file and thus skip it on all platforms. That's my preference.

OK with me.

[wholmgren]

I think this is ok to merge.

[wholmgren]

Guessing the installation failure is related to NREL/pysam#68

CI installs nrel-pysam using pip, so it's not clear to me how they're related.