fix util.chain type hint

trimesh/util.py

@@ -1389,12 +1389,15 @@ class : Optional[Callable]
     raise ValueError("Unable to extract class of name " + name)
 
 
-def chain(*args: Iterable[Union[Iterable[Any], Any]]) -> List[Any]:
+def chain(*args: Iterable[Union[Iterable[Any], Any, None]]) -> List[Any]:
     """
     A less principled version of `list(itertools.chain(*args))` that accepts
     non-iterable values and filters `None` and returns a list
     rather than yielding values.
 
+    If all passed values are iterables, this will return identical
+    results to `list(itertools.chain(*args))`:
+
     In [5]: list(itertools.chain([1,2], [3]))
     Out[5]: [1, 2, 3]
 
@@ -1408,7 +1411,6 @@ def chain(*args: Iterable[Union[Iterable[Any], Any]]) -> List[Any]:
       ----> 1 list(itertools.chain([1,2], [3], 4))
       TypeError: 'int' object is not iterable
 
-
     Parameters
     -----------
     args