Suppress mypy warning (#1410)

Warning is:
```
cmd2/argparse_custom.py: note: In member "_print_message" of class "Cmd2ArgumentParser":
cmd2/argparse_custom.py:1364:44: error: Argument 2 of "_print_message" is incompatible with supertype "ArgumentParser"; supertype defines the argument type as "SupportsWrite[str] | None"  [override]
cmd2/argparse_custom.py:1364:44: note: This violates the Liskov substitution principle
cmd2/argparse_custom.py:1364:44: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
```

Author: tleonhardt

cmd2/argparse_custom.py

@@ -1361,7 +1361,7 @@ def format_help(self) -> str:
         # determine help from format above
         return formatter.format_help() + '\n'
 
-    def _print_message(self, message: str, file: Optional[IO[str]] = None) -> None:
+    def _print_message(self, message: str, file: Optional[IO[str]] = None) -> None:  # type: ignore[override]
         # Override _print_message to use style_aware_write() since we use ANSI escape characters to support color
         if message:
             if file is None: