schema: fix type for max_elements

Those functions can return None, fix their expected type.

Closes: #102
Signed-off-by: Stefan Gula <steweg@gmail.com>
Signed-off-by: Samuel Gauthier <samuel.gauthier@6wind.com>

Author: samuel-gauthier

libyang/schema.py

@@ -1382,7 +1382,7 @@ def defaults(self) -> Iterator[Union[None, bool, int, str, float]]:
             else:
                 yield val
 
-    def max_elements(self) -> int:
+    def max_elements(self) -> Optional[int]:
         return (
             self.cdata_leaflist.max
             if self.cdata_leaflist.max != (2**32 - 1)
@@ -1506,7 +1506,7 @@ def uniques(self) -> Iterator[List[SNode]]:
                 nodes.append(SNode.new(self.context, node))
             yield nodes
 
-    def max_elements(self) -> int:
+    def max_elements(self) -> Optional[int]:
         return self.cdata_list.max if self.cdata_list.max != (2**32 - 1) else None
 
     def min_elements(self) -> int: