refactor: Set handler's name

Author: pawamoy

src/mkdocstrings_handlers/python/handler.py

@@ -53,6 +53,8 @@ def chdir(path: str) -> Iterator[None]:  # noqa: D103
 class PythonHandler(BaseHandler):
     """The Python handler class."""
 
+    name = "python"
+    """The handler's name."""
     domain: str = "py"  # to match Sphinx's default domain
     """The cross-documentation domain/language for this handler."""
     enable_inventory: bool = True