Add pytest to requirements.txt.
Add pytest to requirements.txt.

pytest is a lot nicer to use than the default python testing library
('unittest') and pretty popular, so let's use that.

wakermahmud/sync-engine,nylas/sync-engine,Eagles2F/sync-engine,jobscore/sync-engine,closeio/nylas,PriviPK/privipk-sync-engine,PriviPK/privipk-sync-engine,PriviPK/privipk-sync-engine,closeio/nylas,Eagles2F/sync-engine,gale320/sync-engine,ErinCall/sync-engine,closeio/nylas,gale320/sync-engine,EthanBlackburn/sync-engine,jobscore/sync-engine,nylas/sync-engine,gale320/sync-engine,gale320/sync-engine,PriviPK/privipk-sync-engine,rmasters/inbox,closeio/nylas,nylas/sync-engine,ErinCall/sync-engine,rmasters/inbox,wakermahmud/sync-engine,wakermahmud/sync-engine,ErinCall/sync-engine,EthanBlackburn/sync-engine,nylas/sync-engine,jobscore/sync-engine,Eagles2F/sync-engine,Eagles2F/sync-engine,Eagles2F/sync-engine,ErinCall/sync-engine,EthanBlackburn/sync-engine,rmasters/inbox,EthanBlackburn/sync-engine,gale320/sync-engine,ErinCall/sync-engine,rmasters/inbox,wakermahmud/sync-engine,PriviPK/privipk-sync-engine,jobscore/sync-engine,wakermahmud/sync-engine,EthanBlackburn/sync-engine