Stop using asyncio.get_event_loop()

It is deprecated in newer Python versions.
Closes #4013.

readthedocs/misc/changelog.rst

@@ -2088,7 +2088,7 @@ the scenes! This means you're now able to do both of the following:
     async def main():
       await client.send_message('me', 'Hello!')
 
-    asyncio.get_event_loop().run_until_complete(main())
+    asyncio.run(main())
 
     # ...can be rewritten as:
 

readthedocs/misc/compatibility-and-convenience.rst

@@ -161,19 +161,17 @@ just get rid of ``telethon.sync`` and work inside an ``async def``:
 
             await client.run_until_disconnected()
 
-    loop = asyncio.get_event_loop()
-    loop.run_until_complete(main())
+    asyncio.run(main())
 
 
-The ``telethon.sync`` magic module simply wraps every method behind:
+The ``telethon.sync`` magic module essentially wraps every method behind:
 
 .. code-block:: python
 
-    loop = asyncio.get_event_loop()
-    loop.run_until_complete(main())
+    asyncio.run(main())
 
-So that you don't have to write it yourself every time. That's the
-overhead you pay if you import it, and what you save if you don't.
+With some other tricks, so that you don't have to write it yourself every time.
+That's the overhead you pay if you import it, and what you save if you don't.
 
 Learning
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