Support Windows 7 with Python 3.8

network_monitor.py

@@ -1,4 +1,4 @@
-"""Windows ping spike monitor. Python 3.10+, standard library only."""
+"""Windows ping spike monitor. Python 3.8+, standard library only."""
 from __future__ import annotations
 
 import argparse
@@ -376,7 +376,11 @@ def main():
         log.write("error", f"Error: {error}", error=str(error))
     finally:
         stop.set()
-        executor.shutdown(wait=True, cancel_futures=True)
+        # Python 3.8 has no shutdown(cancel_futures=...). There is at most
+        # one submitted trace; cancel it if queued, or let stop end it if running.
+        if pending is not None:
+            pending.cancel()
+        executor.shutdown(wait=True)
         log.write("stop", "Monitor stopped.", samples=samples)
         log.file.close()
     return 1 if failed else 0