typing: MultiDict.update accepts Iterable Mapping values.

MultiDict.__init__ correctly represents it, however the update was
missing the iterable variant.

Also, flipped the order of Iterable[V] and V fixes PyCharm's naive
matching for generics. Mypy would accept both versions, however PyCharm
would only try to infer Mapping V as the List. By listing it second it
will first try to map to the Iterable[V].

CHANGES.rst

@@ -8,6 +8,8 @@ Unreleased
 -   Handle multiple tokens in ``Connection`` header when routing
     WebSocket requests. :issue:`2131`
 -   Set the debugger pin cookie secure flag when on https. :pr:`2150`
+-   Fix type annotation for ``MultiDict.update`` to accept iterable values
+    :pr:`2142`
 
 
 Version 2.0.1

src/werkzeug/datastructures.pyi

@@ -115,7 +115,7 @@ class MultiDict(TypeConversionDict[K, V]):
     def __init__(
         self,
         mapping: Optional[
-            Union[Mapping[K, Union[V, Iterable[V]]], Iterable[Tuple[K, V]]]
+            Union[Mapping[K, Union[Iterable[V], V]], Iterable[Tuple[K, V]]]
         ] = None,
     ) -> None: ...
     def __getitem__(self, item: K) -> V: ...
@@ -141,7 +141,7 @@ class MultiDict(TypeConversionDict[K, V]):
     @overload
     def to_dict(self, flat: Literal[False]) -> Dict[K, List[V]]: ...
     def update(  # type: ignore
-        self, mapping: Union[Mapping[K, V], Iterable[Tuple[K, V]]]
+        self, mapping: Union[Mapping[K, Union[Iterable[V], V]], Iterable[Tuple[K, V]]]
     ) -> None: ...
     @overload
     def pop(self, key: K) -> V: ...