add python 2.7 deprecation warning
add python 2.7 deprecation warning

In a future version of streamlink only Python 3.5 and above will be
supported.

chhe/streamlink,beardypig/streamlink,chhe/streamlink,streamlink/streamlink,gravyboat/streamlink,bastimeyer/streamlink,beardypig/streamlink,streamlink/streamlink,melmorabity/streamlink,bastimeyer/streamlink,gravyboat/streamlink,melmorabity/streamlink