Multiples of 3 and 5: set based solution.

[ShivnarenSrinivasan]

Variable names are more descriptive than in the other files (in the directory)—happy to refactor the others, if required.

Describe your change:

• 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}.

[poyea]

Please see and wait for #5496.

[ShivnarenSrinivasan]

Please see and wait for #5496.

This issue seems to be merged; not too sure what the next step from my end is?
Should I pull master into my branch?

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[cclauss]

In preparation for that denial-of-service attack that is Hacktoberfest, I am closing all pull requests that do not have passing tests.