Python Digital root

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

[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}.

Aug 13 '22 11:08 meg-1

@poyea @cclauss , hi! thank you for your recommendations. i have closed, and recreated the previous pull requests!

Aug 17 '22 14:08 meg-1

@Kush1101 hi! could you please check out this pull request?

Aug 27 '22 12:08 meg-1

@Kush1101 hi! could you please check out this pull request?

Sep 03 '22 12:09 meg-1

Please run psf/black on the code as discussed in CONTRIBUTING.md.

Sep 14 '22 15:09 cclauss

In preparation for that denial-of-service attack that is Hacktoberfest, I am closing all pull requests that have failing tests.

Oct 01 '22 08:10 cclauss