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/queues.py

@@ -5,7 +5,6 @@ __all__ = ['Queue', 'PriorityQueue', 'LifoQueue', 'QueueFull', 'QueueEmpty']
 import collections
 import heapq
 
-from . import compat
 from . import events
 from . import locks
 from .coroutines import coroutine
@@ -251,9 +250,3 @@ class LifoQueue(Queue):
 
     def _get(self):
         return self._queue.pop()
-
-
-if not compat.PY35:
-    JoinableQueue = Queue
-    """Deprecated alias for Queue."""
-    __all__.append('JoinableQueue')