Issue #28544: Implement asyncio.Task in C.

This implementation provides additional 10-20% speed boost for
asyncio programs.

The patch also fixes _asynciomodule.c to use Arguments Clinic, and
makes '_schedule_callbacks' an overridable method (as it was in 3.5).

Lib/asyncio/base_events.py

@@ -57,7 +57,7 @@ _FATAL_ERROR_IGNORE = (BrokenPipeError,
 
 def _format_handle(handle):
     cb = handle._callback
-    if inspect.ismethod(cb) and isinstance(cb.__self__, tasks.Task):
+    if isinstance(getattr(cb, '__self__', None), tasks.Task):
         # format the task
         return repr(cb.__self__)
     else: