Hacktoberfest: adding doctest to radix_sort.py file (#2779)

* adding doctest to radix_sort.py file

* fixup! Format Python code with psf/black push

* Update radix_sort.py

* Update radix_sort.py

* fixup! Format Python code with psf/black push

* Update radix_sort.py

* line

* fix tests

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: John Law <johnlaw.po@gmail.com>