Fix mypy in #2684

[poyea]

Describe your change:

Fix mypy errors made in #2684.

• 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}.

[ghost]

Pull Request Report

@poyea Hello! I'm a bot made to check all the pull request Python files. First of all, I want to say thank you for your time and interest in this project and for opening a pull request. There seems to be missing requirements in some of the Python files submitted in this pull request. Please read through the report and make the necessary changes. You can take a look at the relevant links provided after the report.

What are node paths? 🔽

The report contains headings and a checklist where the items are paths to the class/function/parameter where the requirement is missing. Node paths are double colon :: separated names and can be in any of the following format:

• Class path: [file_name]::[class_name]
• Function path: [file_name]::[function_name]
• Function parameter path: [file_name]::[function_name]::[parameter_name]
• Method path: [file_name]::[class_name]::[function_name]
• Method parameter path: [file_name]::[class_name]::[function_name]::[parameter_name]

Following functions require tests [doctest/unittest/pytest]:

• backtracking/all_combinations.py::create_all_state
• backtracking/all_combinations.py::print_all_state
• backtracking/all_permutations.py::generate_all_permutations
• backtracking/all_permutations.py::create_state_space_tree
• backtracking/n_queens.py::isSafe
• backtracking/n_queens.py::solve
• backtracking/n_queens.py::printboard
• backtracking/rat_in_maze.py::run_maze
• backtracking/sum_of_subsets.py::generate_sum_of_subsets_soln
• backtracking/sum_of_subsets.py::create_state_space_tree

Following class/functions/parameters require descriptive names:

• backtracking/all_combinations.py::generate_all_combinations::n
• backtracking/all_combinations.py::generate_all_combinations::k
• backtracking/rat_in_maze.py::run_maze::i
• backtracking/rat_in_maze.py::run_maze::j

Relevant links 🔽

• Contributing guidelines
• Project Euler solution guidelines
• Type hints
• doctest
• unittest
• pytest

[dhruvmanila]

mypy passes for all files ❤️
Awesome job! Thanks for doing this 😄