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 ;-)
diff --git a/Lib/asyncio/futures.py b/Lib/asyncio/futures.py
index b2e57ef..7b6204a 100644
--- a/Lib/asyncio/futures.py
+++ b/Lib/asyncio/futures.py
@@ -9,7 +9,6 @@
 import traceback
 
 from . import base_futures
-from . import compat
 from . import events
 
 
@@ -238,8 +237,7 @@
         assert self.done(), "yield from wasn't used with future"
         return self.result()  # May raise too.
 
-    if compat.PY35:
-        __await__ = __iter__ # make compatible with 'await' expression
+    __await__ = __iter__ # make compatible with 'await' expression
 
 
 # Needed for testing purposes.