application: fix DeprecationWarning (prompt-toolkit#1798)

Author: doronz88

src/prompt_toolkit/application/application.py

@@ -958,7 +958,13 @@ def run_in_thread() -> None:
             # See whether a loop was installed already. If so, use that. That's
             # required for the input hooks to work, they are installed using
             # `set_event_loop`.
-            loop = asyncio.get_event_loop()
+            if sys.version_info < (3, 10):
+                loop = asyncio.get_event_loop()
+            else:
+                try:
+                    loop = asyncio.get_running_loop()
+                except RuntimeError:
+                    loop = asyncio.new_event_loop()
         except RuntimeError:
             # No loop installed. Run like usual.
             return asyncio.run(coro)