Flush python 3.6 requirement

[edoddridge]

The github actions yaml had the python version pinned to 3.6. This version is no longer available. Pinned version has been removed. If everything passes, then there's no need to pin the version.

[codecov[bot]]

Codecov Report

Merging #245 (9251ad5) into master (6cd1727) will increase coverage by 0.16%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
+ Coverage   74.10%   74.27%   +0.16%     
==========================================
  Files          20       20              
  Lines        1425     1407      -18     
  Branches       89       89              
==========================================
- Hits         1056     1045      -11     
+ Misses        315      307       -8     
- Partials       54       55       +1     

Impacted Files	Coverage Δ
src/model_main.f90	80.53% <0.00%> (-3.54%)	:arrow_down:
src/barotropic_mode.f90	77.70% <0.00%> (-0.56%)	:arrow_down:
src/io.f90	79.50% <0.00%> (+0.50%)	:arrow_up:
src/time_stepping.f90	64.19% <0.00%> (+5.25%)	:arrow_up:

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[edoddridge]

All looks good. The jitter in the codecov results is odd - that feature is still not working as desired (see #242).