Add Python 3.9 to CI runs and use it for the experimental run

[pnuu]

Python 3.9 wasn't used to run the unit tests. This'll add it, and swap the experimental test runs to it.

• Tests passed

[codecov]

Codecov Report

Merging #396 (4ced7ae) into main (ee7c1b9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #396   +/-   ##
=======================================
  Coverage   93.80%   93.80%           
=======================================
  Files          65       65           
  Lines       11038    11038           
=======================================
  Hits        10354    10354           
  Misses        684      684           

Flag	Coverage Δ
unittests	93.80% <ø> (ø)

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[coveralls]

Coverage increased (+0.002%) to 93.606% when pulling 4ced7ae on pnuu:ci-python-3.9 into ee7c1b9 on pytroll:main.

[djhoese]

@gerritholl had brought up that numpy will drop 3.7 support by the end of the year. Should we drop 3.7 testing now?

[djhoese]

Thoughts on dropping Python 3.7 in the tests @gerritholl @mraspaud @pnuu ?

[gerritholl]

Thoughts on dropping Python 3.7 in the tests

Fine with me, although if we follow NEP 29 strictly, then we should wait for 2021-12-26 to do so (or for the first release after this date).

[djhoese]

Ok so maybe we just merge this?

[gerritholl]

I think so. Maybe by Christmas we can add Python 3.10 tests and drop Python 3.7 at the same time.