digital root method

[meg-1]

Describe your change:

added the 'digital_root' method to maths/sum_of_digits.py. a digital root is: https://en.wikipedia.org/wiki/Digital_root

• [x] Add an algorithm?
• [ ] Fix a bug or typo in an existing algorithm?
• [ ] Documentation change?

Checklist:

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

[meg-1]

@poyea , sorry to disturb, the pre-commits in my pull requests keep failing although everything seems alright with the code. the workflow keeps telling me to add blank lines, but it never lets the checks go through even when i add them. could you please reccomend something to fix this issue?

[poyea]

FAILED arithmetic_analysis/in_static_equilibrium.py::arithmetic_analysis.in_static_equilibrium.polar_force can be ignored. See https://github.com/TheAlgorithms/Python/issues/6170

[meg-1]

@Kush1101 could you please check out this pull request?

[meg-1]

@Kush1101 could you please check out this pull request?

[meg-1]

@poyea , hi! i would like to ask if the error shown after running my file in this commit is on my side, or on the repository's?? here's an attatched screen.

[cclauss]

Please rebase this pull request.