Add mypy

[andersonberg]

Fixes #145
This PR will add mypy and fix type errors. I was running mypy locally this way: $ mypy src/arche --ignore-missing-imports.

[codecov]

Codecov Report

Merging #152 into master will decrease coverage by 0.02%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
- Coverage   79.31%   79.29%   -0.03%     
==========================================
  Files          22       22              
  Lines        1615     1618       +3     
  Branches      284      284              
==========================================
+ Hits         1281     1283       +2     
- Misses        288      289       +1     
  Partials       46       46

Impacted Files	Coverage Δ
src/arche/rules/result.py	87.85% <100%> (ø)	⬆️
src/arche/tools/api.py	64.64% <100%> (ø)	⬆️
src/arche/tools/helpers.py	100% <100%> (ø)	⬆️
src/arche/rules/others.py	100% <100%> (ø)	⬆️
src/arche/rules/duplicates.py	100% <100%> (ø)	⬆️
src/arche/tools/schema.py	87.95% <100%> (ø)	⬆️
src/arche/report.py	97.72% <100%> (ø)	⬆️
src/arche/data_quality_report.py	77.52% <100%> (ø)	⬆️
src/arche/readers/schema.py	100% <100%> (ø)	⬆️
src/arche/readers/items.py	87.06% <66.66%> (-0.76%)	⬇️
... and 1 more

---

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[codecov]

Codecov Report

Merging #152 into master will decrease coverage by 0.03%.
The diff coverage is 95.08%.

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
- Coverage   81.79%   81.76%   -0.04%     
==========================================
  Files          23       23              
  Lines        1637     1634       -3     
  Branches      291      290       -1     
==========================================
- Hits         1339     1336       -3     
  Misses        246      246              
  Partials       52       52

Impacted Files	Coverage Δ
src/arche/rules/json_schema.py	100% <ø> (ø)	⬆️
src/arche/rules/result.py	88.31% <100%> (ø)	⬆️
src/arche/tools/api.py	64.64% <100%> (ø)	⬆️
src/arche/tools/bitbucket.py	100% <100%> (ø)	⬆️
src/arche/tools/helpers.py	100% <100%> (ø)	⬆️
src/arche/rules/others.py	100% <100%> (ø)	⬆️
src/arche/rules/duplicates.py	100% <100%> (ø)	⬆️
src/arche/tools/schema.py	87.95% <100%> (ø)	⬆️
src/arche/readers/items.py	88.07% <100%> (+0.24%)	⬆️
src/arche/arche.py	84.89% <100%> (ø)	⬆️
... and 4 more

---

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[ejulio]

Just not sure about the benefits, mostly because:

• Only a few variables/attributes are typed
• There are some #type: ignore

[manycoding]

What's the error in mypy for every line with # type: ignore?

I agree with @ejulio and think it's better to either not have # type: ignore at all or don't use mypy.

Last thing - adding mypy check to tox like here https://github.com/scrapinghub/arche/compare/mypy

[manycoding]

Add this check to travis.yml

[manycoding]

Awesome.

[manycoding]

I cleaned some redundant castings and made sure it works in travis. Good work, I am merging.