Fix the PopIt-Python line in requirements.txt
Fix the PopIt-Python line in requirements.txt

The output from "pip freeze" isn't usable under some circumstances:

   https://github.com/pypa/pip/issues/1083

openstate/yournextrepresentative,mysociety/yournextrepresentative,mysociety/yournextmp-popit,mysociety/yournextrepresentative,datamade/yournextmp-popit,DemocracyClub/yournextrepresentative,mhl/yournextmp-popit,datamade/yournextmp-popit,mysociety/yournextrepresentative,neavouli/yournextrepresentative,openstate/yournextrepresentative,YoQuieroSaber/yournextrepresentative,neavouli/yournextrepresentative,mysociety/yournextmp-popit,mhl/yournextmp-popit,YoQuieroSaber/yournextrepresentative,mysociety/yournextmp-popit,mysociety/yournextrepresentative,DemocracyClub/yournextrepresentative,openstate/yournextrepresentative,neavouli/yournextrepresentative,datamade/yournextmp-popit,datamade/yournextmp-popit,YoQuieroSaber/yournextrepresentative,neavouli/yournextrepresentative,openstate/yournextrepresentative,YoQuieroSaber/yournextrepresentative,mhl/yournextmp-popit,YoQuieroSaber/yournextrepresentative,mysociety/yournextmp-popit,mysociety/yournextrepresentative,DemocracyClub/yournextrepresentative,openstate/yournextrepresentative,neavouli/yournextrepresentative,mysociety/yournextmp-popit,datamade/yournextmp-popit