Add explicit support and build tests for Python 3.9

[alurban]

This PR adds explicit support and build tests for python-3.9. This is needed to get out a new minor release, which will allow us to distribute hveto to Conda platforms running the new version of python, see conda-forge/hveto-feedstock#17.

[codecov]

Codecov Report

Merging #147 (e449926) into master (ac66e0a) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
+ Coverage   36.27%   36.51%   +0.24%     
==========================================
  Files          13       13              
  Lines        1533     1534       +1     
==========================================
+ Hits          556      560       +4     
+ Misses        977      974       -3     

Flag	Coverage Δ
Linux	36.51% <100.00%> (+0.24%)	⬆️
python36	36.30% <100.00%> (+0.24%)	⬆️
python37	36.30% <100.00%> (+0.24%)	⬆️
python38	36.53% <100.00%> (+0.24%)	⬆️
python39	36.53% <100.00%> (?)

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Impacted Files	Coverage Δ
hveto/__init__.py	100.00% <ø> (ø)
hveto/__main__.py	11.40% <ø> (+0.22%)	⬆️
hveto/cli/cache_events.py	16.99% <ø> (+0.49%)	⬆️
hveto/cli/trace.py	43.48% <ø> (+2.17%)	⬆️
hveto/config.py	32.56% <ø> (ø)
hveto/core.py	26.25% <ø> (ø)
hveto/html.py	56.41% <ø> (ø)
hveto/plot.py	53.67% <ø> (ø)
hveto/segments.py	92.68% <ø> (ø)
hveto/triggers.py	66.18% <ø> (ø)
... and 4 more

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[jrsmith02]

My apologies for the delay in responding. Thank you so much Alex for adding explicit Python 3.9 support to hveto. LGTM.

[alurban]

No worries, thanks for taking a look @jrsmith02! I'll put out a release later today.