ref(worker): Amend return type of submit and flush to accomodate for coroutines

Coroutines do not return None, therefore it is necessary to consider this in the callback parameter of the worker. Previously,
only callbacks with return Type None were accepted.

GH-4581

Author: srothh

sentry_sdk/worker.py

@@ -259,7 +259,7 @@ async def flush_async(self, timeout: float, callback: Optional[Any] = None) -> N
             await self._wait_flush(timeout, callback)
         logger.debug("background worker flushed")
 
-    def submit(self, callback: Callable[[], None]) -> bool:
+    def submit(self, callback: Callable[[], Any]) -> bool:
         self._ensure_task()
 
         try: