refactored into a more modern package

[xrotwang]

• switch to pytest for testing
• enable combined coverage reports across python versions (including branch coverage)
• added contributing guidelines
• made sure the short tutorial in the README works

[codecov-io]

Codecov Report

Merging #24 into master will decrease coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
- Coverage   99.24%   99.23%   -0.01%     
==========================================
  Files           9        6       -3     
  Lines         395      263     -132     
  Branches        0       56      +56     
==========================================
- Hits          392      261     -131     
+ Misses          3        0       -3     
- Partials        0        2       +2

Impacted Files	Coverage Δ
src/segments/errors.py	100% <ø> (ø)
src/segments/tokenizer.py	100% <ø> (ø)
src/segments/util.py	100% <ø> (ø)
src/segments/tree.py	96.66% <ø> (ø)
src/segments/cli.py	100% <100%> (ø)
src/segments/__init__.py	100% <100%> (ø)

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[xrotwang]

Note that the decreased test coverage is due to switching to measure branch coverage as well.

[bambooforest]

That was the only thing I was curious about. The rest looks "modern". Thanks for this!

[xflr6]

@bambooforest: next time give me some minutes to complete my review :)

[bambooforest]

@xflr6 -- my fault, didn't see you on this, but good thing you are!