Update README for Python 3 support
Update README for Python 3 support

This topic branch resolves https://github.com/pydy/pydy/issues/38.

oliverlee/pydy,Shekharrajak/pydy,skidzo/pydy,Shekharrajak/pydy,Shekharrajak/pydy,skidzo/pydy,Shekharrajak/pydy,skidzo/pydy,oliverlee/pydy,skidzo/pydy,oliverlee/pydy