Improve MultiValueDict/QueryDict dict() return (#899)

According to Django documentation and code, the `MultiValueDict.dict()` method only returns singular values for each key, never a list.

https://docs.djangoproject.com/en/dev/ref/request-response/#django.http.QueryDict.dict

https://github.com/django/django/blob/3de787a70b6c140a3b1289a8ce12b17ebcb8a200/django/utils/datastructures.py#L215-L217

django-stubs/utils/datastructures.pyi

@@ -43,7 +43,7 @@ class MultiValueDict(MutableMapping[_K, _V]):
     def setlistdefault(self, key: _K, default_list: Optional[List[_V]] = ...) -> List[_V]: ...
     def appendlist(self, key: _K, value: _V) -> None: ...
     def lists(self) -> Iterable[Tuple[_K, List[_V]]]: ...
-    def dict(self) -> Dict[_K, Union[_V, List[_V]]]: ...
+    def dict(self) -> Dict[_K, _V]: ...
     def copy(self: _D) -> _D: ...
     # These overrides are needed to convince mypy that this isn't an abstract class
     def __delitem__(self, item: _K) -> None: ...