Add 3.8 as a supported Python version in docs

[jGaboardi]

This PR adds 3.8 to the documented supported Python versions.

See #392

[codecov]

Codecov Report

Merging #422 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #422   +/-   ##
=======================================
  Coverage   98.07%   98.07%           
=======================================
  Files           5        5           
  Lines        1770     1770           
  Branches      220      220           
=======================================
  Hits         1736     1736           
  Misses         14       14           
  Partials       20       20

Impacted Files	Coverage Δ
spaghetti/util.py	98.95% <0%> (ø)	⬆️
spaghetti/tests/test_network_api.py	100% <0%> (ø)	⬆️
spaghetti/tests/test_network.py	100% <0%> (ø)	⬆️
spaghetti/network.py	95.39% <0%> (ø)	⬆️

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