Feat: Add smallest_multiple

[domix80]

Describe your change:

• [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.
• [ ] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• [ ] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[tianyizheng02]

This is the same problem as problem 5 from Project Euler, so perhaps this would better serve as an alternate solution to that problem?

[domix80]

Please remove smallest_multiple.py and keep sol3.py

[tianyizheng02]

Make sure you follow the guidelines for submitting a Project Euler solution

[domix80]

Fix TCs for sol3.py. Please discard smallest_multiple.py in math folder.

[tianyizheng02]

Why not remove smallest_multiple.py yourself for the PR?

[dhruvmanila]

Thank you for taking the time to contribute but I'm sorry to say that we're not accepting solutions to already solved problems currently. You can try submitting solution for any unsolved problems :)