Add bin/test_external_imports.py
Add bin/test_external_imports.py

This script will make sure no external libraries are being imported with 'from
sympy import *' that aren't supposed to be. This will avoid issues in the
future like the one fixed in the previous commit where tensorflow was imported
alongside SymPy.

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy