Add mypy to style guidelines

[purva-thakre]

Noticed mypy was missing from the style guidelines in docs.

[github-actions]

👈 Launch a binder notebook on branch unitaryfund/mitiq/mypy_in_doc

[codecov]

Codecov Report

Merging #1841 (b4b1936) into master (0716714) will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1841      +/-   ##
==========================================
- Coverage   98.54%   98.44%   -0.10%     
==========================================
  Files          73       73              
  Lines        3357     3400      +43     
==========================================
+ Hits         3308     3347      +39     
- Misses         49       53       +4     

see 3 files with indirect coverage changes

[purva-thakre]

This is ready. Not sure why codecov is failing.

I did not make any changes to the files that codecov thinks are not covered by tests.

[natestemen]

Good thinking adding this. Always good to have our processes documented thoroughly.