asyncio: Remove asyncio/compat.py (#4606)

The asyncio/compat.py file was written to support Python < 3.5 and
Python < 3.5.2. But Python 3.5 doesn't accept bugfixes anymore, only
security fixes. There is no more need to backport bugfixes to Python
3.5, and so no need to have a single code base for Python 3.5, 3.6
and 3.7.

Say hello (again) to "async" and "await", who became real keywords in
Python 3.7 ;-)

Lib/asyncio/streams.py

@@ -12,7 +12,6 @@ if hasattr(socket, 'AF_UNIX'):
     __all__.extend(['open_unix_connection', 'start_unix_server'])
 
 from . import coroutines
-from . import compat
 from . import events
 from . import protocols
 from .coroutines import coroutine