roster: fix overwriting old saves on Windows

pqcli/roster.py

@@ -1,3 +1,4 @@
+import os
 import logging
 import pickle
 import typing as T
@@ -29,6 +30,8 @@ class Roster:
         tmp_path.parent.mkdir(parents=True, exist_ok=True)
         tmp_path.write_bytes(pickle.dumps(self.players))
         if self.path.exists():
+            if old_path.exists():
+                old_path.unlink()
             self.path.rename(old_path)
         tmp_path.rename(self.path)