fix: avoid a dict-changed-size error

Windows 3.12 failed occasionally with this error:

```
      @panopticon()
      def stop(self) -> None:
          """Stop this Tracer."""
          if not self.sysmon_on:
              # In forking situations, we might try to stop when we are not
              # started.  Do nothing in that case.
              return
          assert sys_monitoring is not None
          sys_monitoring.set_events(self.myid, 0)
  >       for code in self.local_event_codes.values():
  E       RuntimeError: dictionary changed size during iteration
```

By setting self.sysmon_on=False before iteration, we lessen the chance
of adding an item to the dict during the iteration.

Author: nedbat

coverage/sysmon.py

@@ -251,11 +251,11 @@ def stop(self) -> None:
             return
         assert sys_monitoring is not None
         sys_monitoring.set_events(self.myid, 0)
+        self.sysmon_on = False
         for code in self.local_event_codes.values():
             sys_monitoring.set_local_events(self.myid, code, 0)
         self.local_event_codes = {}
         sys_monitoring.free_tool_id(self.myid)
-        self.sysmon_on = False
 
     @panopticon()
     def post_fork(self) -> None: