implement fizzbuzz

[ardge-snowchung]

Goal

Implement the fizzbuzz(n) function in fizzbuzz.py so the existing test suite passes.

Acceptance Criteria

• multiples of 3 -> "Fizz", of 5 -> "Buzz", of both -> "FizzBuzz", else the number as a string.
• python3 -m unittest passes from the repo root.
• Change scoped to fizzbuzz.py; no stray files. Base branch is main.