Python 3.9 compatibility

[stennie]

Test for Python 3.9 compatibility

Release notes: https://docs.python.org/3.9/whatsnew/3.9.html

Expected 3.9.0 final release date: 2020-10-05

[stennie]

Python 3.9 is only available as 3.9-dev in Travis: https://docs.travis-ci.com/user/languages/python/#specifying-python-versions.

[stennie]

matplotib installation apparently fails because there are no numpy wheels for 3.9 yet: numpy/numpy#17482

numpy installs in 3.9 but bails with a runtime error on import:

    RuntimeError: Polyfit sanity test emitted a warning, most likely due to using a buggy Accelerate backend. If you compiled yourself, see site.cfg.example for information. Otherwise report this to the vendor that provided NumPy.
    RankWarning: Polyfit may be poorly conditioned

Looks like it is a few weeks too early to add Python 3.9 to testing.