ci: add mypy configuration and pre-commit hook

[redeboer]

Closes #33

See also #47

[redeboer]

Draft for now because we need to see what configurations to add
https://mypy.readthedocs.io/en/stable/config_file.html

I'm not sure if we should add yet another config file, in this case mypy.ini. So I would prefer to add the mypy configuration to the tox.ini file. See this discussion though... python/mypy#2761.

[codecov]

Codecov Report

Merging #48 into master will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   83.07%   83.04%   -0.04%     
==========================================
  Files          24       24              
  Lines        2618     2618              
==========================================
- Hits         2175     2174       -1     
- Misses        443      444       +1     

Impacted Files	Coverage Δ
tensorwaves/interfaces.py	100.00% <100.00%> (ø)
...ensorwaves/expertsystem/state/conservationrules.py	84.59% <0.00%> (-0.24%)	⬇️

[spflueger]

Good. We should keep in mind to write a small section in the docs about this as well, once we include this officially. Specifically how to config vs code.

[redeboer]

Indeed. An attempt to improve the contribute pages was drafted in ComPWA/pycompwa#85, but that PR should probably be migrated to tensorwaves. Then the instructions can be simplified, we now only use:

• formatter: black
• linting: pylint + flake8 (+ extensions)
• static type checker: mypy

All of that is automatised through pre-commit and Travis CI, just need to point out those four tools and their corresponding configs. The configs are important because they represent our (if necessary, evolving) coding conventions, so those conventions don't need to be written out in the documentation.

Related to that, as you say, we may want to highly recommend contributors to work with VS code. There is currently no such assumption in the documentation.