Split requirements into py2 and py3 files

Since PBR now supports distinguishing between
a requirements file for py2 and a different requirements
file for py3 we should add the support for that separation
so that it is easier to package taskflow.

Change-Id: I3c5faa99a529bcb28aa0276b8d345a26f94b5af3
Merge "Split requirements into py2 and py3 files"