Support Python 3.9 and 3.10

[rhandberg]

We should enable support for Python 3.9 and Python 3.10.

In this process we may drop support for Python 3.6 if that requires too much work. Final decision still pending on that.

[codecov[bot]]

Codecov Report

Merging #34 (84be625) into devel (6c46844) will increase coverage by 0.00%. The diff coverage is 76.00%.

@@           Coverage Diff           @@
##            devel      #34   +/-   ##
=======================================
  Coverage   84.35%   84.35%           
=======================================
  Files          33       33           
  Lines        4359     4366    +7     
=======================================
+ Hits         3677     3683    +6     
- Misses        682      683    +1     

Impacted Files	Coverage Δ
photometry/plots.py	86.08% <33.33%> (-0.45%)	:arrow_down:
photometry/AperturePhotometry/k2p2v2.py	71.94% <42.85%> (-0.22%)	:arrow_down:
photometry/BasePhotometry.py	93.91% <100.00%> (ø)
photometry/halo/halo_photometry.py	80.00% <100.00%> (+0.35%)	:arrow_up:
photometry/prepare.py	84.97% <100.00%> (+0.11%)	:arrow_up:
photometry/utilities.py	96.61% <100.00%> (ø)
photometry/version.py	50.70% <0.00%> (+1.40%)	:arrow_up:

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[rhandberg]

We will drop support for Python 3.6. The status of the PR right now, is that it keeps support for 3.7, but with a slightly older version of astropy.

Python 3.10 is also not working when running under Windows, due to problems with some dependencies, so testing has been disabled for now.

[rhandberg]

I think this is mature enough now that this can be merged in.