Closes #9733: Fix for logging handler flushing warnings in the middle…

… of the build

Summary:
My project was mysteriously dropping warnings (see #9733 for detailed repro) and I realized that it's becaues it imports libraries like airflow or mlflow that set up loggers automatically when they are imported. This causes this handler to flush even though shouldFlush is set to always return False. A simple workaround is to override flush to be a no-op.

Test Plan: Repeat repro steps from #9733 - project now always includes warnings

sphinx/util/logging.py

@@ -171,6 +171,11 @@ def __init__(self) -> None:
     def shouldFlush(self, record: logging.LogRecord) -> bool:
         return False  # never flush
 
+    def flush(self) -> None:
+        # suppress any flushes triggered by importing packages that flush
+        # all handlers at initialization time
+        pass
+
     def flushTo(self, logger: logging.Logger) -> None:
         self.acquire()
         try: