Fix custom HelpFormatter for Python 3.14

python/cpython#132323 passes prefix_chars=
(and other arguments) to the formatter_class, but the custom HelpFormatter only
accepted prog=.

Accept any keyword arguments and pass them on to the parent class.

Fixes tox-dev#3523

Author: The-Compiler

docs/changelog/3523.bugfix.rst

@@ -0,0 +1 @@
+Fix ``TypeError`` for ``HelpFormatter`` with Python 3.14

src/tox/config/cli/parser.py

@@ -95,8 +95,8 @@ def parse_args(  # type: ignore[override] # avoid defining all overloads
 class HelpFormatter(ArgumentDefaultsHelpFormatter):
     """A help formatter that provides the default value and the source it comes from."""
 
-    def __init__(self, prog: str) -> None:
-        super().__init__(prog, max_help_position=30, width=240)
+    def __init__(self, prog: str, **kwargs: Any) -> None:
+        super().__init__(prog, max_help_position=30, width=240, **kwargs)
 
     def _get_help_string(self, action: Action) -> str | None:
         text: str = super()._get_help_string(action) or ""