Add experimental support for async sessions (#4667)

There no plans for this to ever be non-experimental in v1.

telethon/client/auth.py

@@ -552,7 +552,7 @@ class AuthMethods:
         self._authorized = False
 
         await self.disconnect()
-        self.session.delete()
+        await utils.maybe_async(self.session.delete())
         self.session = None
         return True