chore(mypy): Fix before automated tweaks

src/tmuxp/workspace/finders.py:22: error: Incompatible default for argument "extensions" (default has type "None", argument has type "Literal[".yml", ".yaml", ".json"] | list[Literal[".yml", ".yaml", ".json"]]")  [assignment]
src/tmuxp/workspace/finders.py:22: note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True
src/tmuxp/workspace/finders.py:22: note: Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebase

Author: tony

src/tmuxp/workspace/finders.py

@@ -19,7 +19,7 @@
 
 def is_workspace_file(
     filename: str,
-    extensions: t.Union["ValidExtensions", t.List["ValidExtensions"]] = None,
+    extensions: t.Union["ValidExtensions", t.List["ValidExtensions"], None] = None,
 ) -> bool:
     """
     Return True if file has a valid workspace file type.