bpo-40782: Change asyncio.AbstractEventLoop.run_in_executor to be a method not a coroutine (GH-21852)

asyncio.AbstractEventLoop.run_in_executor should be a method that returns an asyncio Future, not an async method.
This matches the concrete implementations, and the documentation better.
(cherry picked from commit 29f8429)

Co-authored-by: James Weaver <james.barrett@bbc.co.uk>

Author: miss-islington

Lib/asyncio/events.py

@@ -280,7 +280,7 @@ def create_task(self, coro, *, name=None):
     def call_soon_threadsafe(self, callback, *args):
         raise NotImplementedError
 
-    async def run_in_executor(self, executor, func, *args):
+    def run_in_executor(self, executor, func, *args):
         raise NotImplementedError
 
     def set_default_executor(self, executor):

Misc/NEWS.d/next/Library/2020-08-13-08-07-25.bpo-40782.aGZqmB.rst

@@ -0,0 +1 @@
+Change the method asyncio.AbstractEventLoop.run_in_executor to not be a coroutine.