Fix missing param default in __text_signature__

Author: johnslavik

Lib/functools.py

@@ -1058,7 +1058,7 @@ def wrapper(*args, **kw):
     funcname = getattr(func, '__name__', 'singledispatch function')
     registry[object] = func
     wrapper.register = register
-    wrapper.register.__text_signature__ = "(cls, func)"  # Hide _role from help().
+    wrapper.register.__text_signature__ = "(cls, func=None)"  # Hide _role from help().
     wrapper.dispatch = dispatch
     wrapper.registry = MappingProxyType(registry)
     wrapper._clear_cache = dispatch_cache.clear