Remove Python 3.6 from test set

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

Closes #491. Python 3.6 reached end of life over the holidays. We can go back to only testing 4 versions, and if we want we can start using features of 3.7+ such as the capture_output flag to subprocess.run

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[codecov-commenter]

Codecov Report

Merging #511 (5b27740) into develop (a14e9bd) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #511      +/-   ##
===========================================
- Coverage    79.06%   79.05%   -0.02%     
===========================================
  Files           45       45              
  Lines         9411     9396      -15     
===========================================
- Hits          7441     7428      -13     
+ Misses        1970     1968       -2     

Impacted Files	Coverage Δ
cmdstanpy/cmdstanpy/stanfit/mle.py	90.09% <0.00%> (-0.29%)	⬇️
a/cmdstanpy/cmdstanpy/cmdstanpy/stanfit/mle.py	90.09% <0.00%> (-0.29%)	⬇️
.../work/cmdstanpy/cmdstanpy/cmdstanpy/stanfit/mle.py	90.09% <0.00%> (-0.29%)	⬇️
cmdstanpy/cmdstanpy/stanfit/vb.py	94.11% <0.00%> (-0.09%)	⬇️
a/cmdstanpy/cmdstanpy/cmdstanpy/stanfit/vb.py	94.11% <0.00%> (-0.09%)	⬇️
...r/work/cmdstanpy/cmdstanpy/cmdstanpy/stanfit/vb.py	94.11% <0.00%> (-0.09%)	⬇️
a/cmdstanpy/cmdstanpy/cmdstanpy/cmdstan_args.py	94.08% <0.00%> (-0.02%)	⬇️
cmdstanpy/cmdstanpy/cmdstan_args.py	94.89% <0.00%> (-0.02%)	⬇️
...work/cmdstanpy/cmdstanpy/cmdstanpy/cmdstan_args.py	94.89% <0.00%> (-0.02%)	⬇️
cmdstanpy/cmdstanpy/stanfit/mcmc.py	92.20% <0.00%> (ø)
... and 3 more

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