Pre-commit: add mypy

[cclauss]

Describe your change:

• Add mypy tests to pre-commit
• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[dhruvmanila]

As mentioned in this comment #4315 (comment), there's going to be problem when running mypy in pre-commit.

The problem with running mypy in pre-commit is that pre-commit runs every hook in its own virtual environment and as mypy needs the information from project dependencies, they need to be installed there as well. pre-commit does provide an option: - additional_dependencies: ['numpy', ...] but that would mean we need to maintain the dependencies in requirements.txt and in pre-commit-config.yaml.

- repo: https://github.com/pre-commit/mirrors-mypy
    rev: v0.812
    hooks:
      - id: mypy
        additional_dependencies: [numpy, beautifulsoup4, requests, ...]

Ref:

• https://pre-commit.com/#config-additional_dependencies
• https://github.com/pre-commit/mirrors-mypy#using-mypy-with-pre-commit (3rd point)

[dhruvmanila]

mypy or any other type checkers should be run in an environment which contains the project dependencies for it to correctly report errors. If we are using any function/class/variable from other modules then mypy will know whether we are passing the correct type of arguments or assigning the correct type to the variable, etc.

This will be easier to understand when we start running mypy in a strict fashion as I am for algorithms-keeper. I faced the same issue when using mypy in pre-commit for the bot. At the end, it was either of the options:

1. Run mypy in pre-commit with specifying the additional_dependencies
2. Run mypy as a local hook in pre-commit
3. Run mypy in CI

[cclauss]

Makes sense.