Fix python version for tests

[jbusecke]

As pointed out correctly by @pbranson here the CI was actually not testing with the python versions specified (and seems to have defaulted to some newer version).
This should correct that behavior.

[codecov-commenter]

Codecov Report

Merging #158 (49aa297) into master (5130514) will increase coverage by 0.30%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
+ Coverage   94.24%   94.55%   +0.30%     
==========================================
  Files          10       10              
  Lines         991      991              
==========================================
+ Hits          934      937       +3     
+ Misses         57       54       -3     

Flag	Coverage Δ
unittests	94.55% <ø> (+0.30%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
gcm_filters/kernels.py	88.88% <0.00%> (+0.58%)	⬆️
tests/test_filter.py	100.00% <0.00%> (+1.06%)	⬆️

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[jbusecke]

With the pinned numcodec version from #157 this was failing for py3.6 (and passing for all others). I have removed the pin to see if that helps to resolve this.
Apologies for not noticing this bug earlier BTW. I hope it does not cause too much disruption. I guess dropping support for python 3.6 could actually be reasonable. EDIT: This actually passed for 3.6 through 3.10.

[NoraLoose]

Nice - thanks for looking into this @jbusecke! I will merge this.