Run `flake8` and `mypy` in the CI

[jond01]

Description

This PR adds the above tools to nox + the CI, including the required modifications to make the sessions pass.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• [x] Test in GitHub CI (see https://github.com/jond01/quantum-viz.js/pull/5)
• [x] Test locally with nox (run all the default sessions)

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] Any dependent changes have been merged and published in downstream modules

[codecov-commenter]

Codecov Report

Merging #48 (ef9ce7e) into main (1cead97) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #48   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           36        36           
=========================================
  Hits            36        36           

Impacted Files	Coverage Δ
quantum-viz/quantum_viz/__init__.py	100.00% <100.00%> (ø)
quantum-viz/quantum_viz/widget.py	100.00% <100.00%> (ø)

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[jond01]

Hi @anpaz @guenp, are these changes accepted? It's just a preliminary work before another PR that will branch out from #44