(Closes #476) Put back testing with python 3.9

[arporter]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.16%. Comparing base (afb9da7) to head (f25b549).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #477   +/-   ##
=======================================
  Coverage   92.16%   92.16%           
=======================================
  Files          86       86           
  Lines       13752    13752           
=======================================
+ Hits        12674    12675    +1     
+ Misses       1078     1077    -1     

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[arporter]

Weirdly, CodeCov is saying that there are indirect coverage changes but I can't see how that has happened. The code in question is actually unreachable so I have removed it.

[arporter]

Ready for review from anyone (@hiker @sergisiso @LonelyCat124).

[LonelyCat124]

Hi @arporter, one small comment as I thinkwe should keep 3.10 testing as we'll move to that once 3.9 is retired.

[arporter]

I've added back testing with 3.10 and all is well. Ready for another look now.

[LonelyCat124]

Looks good to me now, think we can merge