chore: cleanup mypy config

[Saransh-cpp]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes #3489

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[agriyakhetarpal]

Note: when ready, this PR should close #3489, since setting up the type checking configurations was the only remaining task on the list.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.60%. Comparing base (2f9f1e2) to head (a7abf91).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #3853    +/-   ##
=========================================
  Coverage    99.60%   99.60%            
=========================================
  Files          259      259            
  Lines        21284    21480   +196     
=========================================
+ Hits         21200    21396   +196     
  Misses          84       84            

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[agriyakhetarpal]

Thanks, @Saransh-cpp! Question: should we add the from __future__ import annotations statement to every file in the source code? We aren't doing it currently, but I do remember that there is a particular tool that does it and lints files for its absence.

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[Saransh-cpp]

Adding the isort rule blowed up this PR. I'll exclude examples from the isort rule.

in the light of further changes, can review this again when this is ready again