Make PythonLibs find_package python2 specific

On systems with python 3 installed and default, find_package(PythonLibs) will find the python 3 paths and libraries. However, the c++ include structure seems to be different in python 3 and tf2 uses includes that are no longer present or deprecated.

Until the includes are made to be python 3 compliant, we should specify that the version of python found must be python 2.
Merge pull request #8 from cottsay/5adab7ced524f78ccaf7b1e9cef751b4bc1cb31e

Make PythonLibs find_package python2 specific