Add GitHub Actions configuration

[khaeru]

This replaces the Travis and Stickler configuration, reducing by 2 the number of services needed to maintain the repository.

[codecov[bot]]

Codecov Report

Merging #28 into master will increase coverage by 0.34%. The diff coverage is 27.27%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   50.57%   50.92%   +0.34%     
==========================================
  Files          37       34       -3     
  Lines        1568     1467     -101     
==========================================
- Hits          793      747      -46     
+ Misses        775      720      -55     

Impacted Files	Coverage Δ
item/historical/__init__.py	91.66% <ø> (ø)
item/model/plot.py	27.39% <ø> (-0.99%)	:arrow_down:
item/remote/openkapsarc.py	90.00% <0.00%> (ø)
item/tests/test_model.py	0.00% <0.00%> (ø)
item/model/__init__.py	47.05% <100.00%> (+29.41%)	:arrow_up:
item/__init__.py
item/common.py
item/cli.py

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[khaeru]

The "codecov/patch" check failure here is spurious.