Add support for Python 3.8

[hugovk]

Plus:

• Remove Python 2 warning
• use extras_require for test dependencies
• update pre-commit
• lint and test using GitHub Actions

[codecov]

Codecov Report

Merging #32 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #32   +/-   ##
=======================================
  Coverage   41.59%   41.59%           
=======================================
  Files           3        3           
  Lines         327      327           
=======================================
  Hits          136      136           
  Misses        191      191

Impacted Files	Coverage Δ
src/osmviz/animation.py	0% <0%> (ø)	⬆️

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[hugovk]

Before adding pre-commit caching to Travis, tox lint takes 48.47s, whole lint job takes 1m 22s.

With cold cache: 45.03s, 1m 23s.

With warm cache: 8.94s, 43 sec.