Mitigate Shutdown RuntimeError

With the switch to python 3.12 the log started registering a RuntimeError
due to a race condition.

```
systemd[1]: Stopping Fishtest Server port 6543...
pserve[17032]: flush
pserve[17032]: .....................done
pserve[17032]: Exception in thread Thread-740:
pserve[17032]: Traceback (most recent call last):
pserve[17032]:   File "/home/fishtest/.pyenv/versions/3.12.2/lib/python3.12/threading.py", line 1073, in _bootstrap_inner
pserve[17032]:     self.run()
pserve[17032]:   File "/home/fishtest/.pyenv/versions/3.12.2/lib/python3.12/threading.py", line 1431, in run
pserve[17032]:     self.function(*self.args, **self.kwargs)
pserve[17032]:   File "/home/fishtest/fishtest/server/fishtest/rundb.py", line 451, in flush_buffers
pserve[17032]:     self.start_timer()
pserve[17032]:   File "/home/fishtest/fishtest/server/fishtest/rundb.py", line 365, in start_timer
pserve[17032]:     self.timer.start()
pserve[17032]:   File "/home/fishtest/.pyenv/versions/3.12.2/lib/python3.12/threading.py", line 992, in start
pserve[17032]:     _start_new_thread(self._bootstrap, ())
pserve[17032]: RuntimeError: can't create new thread at interpreter shutdown
systemd[1]: Stopped Fishtest Server port 6543.
```

This is the issue opened upstream:
python/cpython#115533

server/fishtest/rundb.py

@@ -361,8 +361,13 @@ def get_run(self, r_id):
                 return None
 
     def start_timer(self):
-        self.timer = threading.Timer(1.0, self.flush_buffers)
-        self.timer.start()
+        try:
+            self.timer = threading.Timer(1.0, self.flush_buffers)
+            self.timer.start()
+        except RuntimeError as e:
+            # RuntimeError that doesn't involve interpreter shutdown
+            if "interpreter shutdown" not in str(e):
+                raise
 
     def buffer(self, run, flush):
         with self.run_cache_lock: