fix multivaluedict init argument

Author: mkurnikov

django-stubs/utils/datastructures.pyi

@@ -12,6 +12,7 @@ from typing import (
     Union,
     overload,
     Iterator,
+    Optional,
 )
 
 from typing_extensions import Literal
@@ -31,9 +32,9 @@ class MultiValueDictKeyError(KeyError): ...
 
 class MultiValueDict(MutableMapping[_K, _V]):
     @overload
-    def __init__(self, key_to_list_mapping: Iterable[Tuple[_K, Union[_V, List[_V]]]] = ...) -> None: ...
+    def __init__(self, key_to_list_mapping: Mapping[_K, Optional[List[_V]]] = ...) -> None: ...
     @overload
-    def __init__(self, key_to_list_mapping: Mapping[_K, Union[_V, List[_V]]] = ...) -> None: ...
+    def __init__(self, key_to_list_mapping: Iterable[Tuple[_K, List[_V]]] = ...) -> None: ...
     def getlist(self, key: _K, default: List[_V] = None) -> List[_V]: ...
     def setlist(self, key: _K, list_: List[_V]) -> None: ...
     def setlistdefault(self, key: _K, default_list: List[_V] = None) -> List[_V]: ...