refactor: move import pytest line of blockchain algs under "main" section. (#2012)

* change doctest line

import doctest is not relevant with algorithms. move it under main section.

* from doctest import testmod

* refactor: move doctest under "main" section

* Update diophantine_equation.py

* Update modular_division.py

Co-authored-by: Christian Clauss <cclauss@me.com>