maths: add Armstrong numbers algorithm

[Ayushi-Jain15]

Describe your change:

Adds armstrong_numbers.py in the maths directory with two functions:

• is_armstrong(): checks if a number is an Armstrong number in O(n)
• get_armstrongs_up_to(): returns all Armstrong numbers up to a given limit
  Both functions include doctests.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• 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 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 description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[diusazzad]

I noticed in your checklist that the Wikipedia URL might be missing. TheAlgorithms usually requires at least one reference link (like Wikipedia) in the docstring of the algorithm. Could you just drop a link in there?