Add/fix mypy type annotations at BFS, DFS in graphs

[mhihasan]

$ mypy --ignore-missing-imports graphs/breadth_first_search.py graphs/breadth_first_search_2.py graphs/depth_first_search.py graphs/depth_first_search_2.py

Success: no issues found in 4 source files

Related Issue: #4052

Describing change

• Add/fix mypy type annoatation at BFS, DFS algo in graphs

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}.

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