Fix setup.py install_requires post targets refactor.

With the target refactor PythonRequirementLibrary is now used rather than
PythonRequirement when generating setup.py install_requires. At present this
field is not populated leading to missing requirements and unisntallable
pants-generated sdist's.

Testing Done:
./build-support/bin/ci.sh

Confirmed setup.py install_requires is now populated.

Reviewed at https://rbcommons.com/s/twitter/r/368/