travis: use pytest to run the tests for coverage
travis: use pytest to run the tests for coverage

gravyboat/streamlink,javiercantero/streamlink,back-to/streamlink,streamlink/streamlink,back-to/streamlink,wlerin/streamlink,beardypig/streamlink,melmorabity/streamlink,wlerin/streamlink,bastimeyer/streamlink,javiercantero/streamlink,streamlink/streamlink,bastimeyer/streamlink,chhe/streamlink,chhe/streamlink,melmorabity/streamlink,gravyboat/streamlink,beardypig/streamlink