Add proxied functions for Promise.

Although there is nothing defined in `Promise` itself, the only
instances of `Promise` are created by the `lazy` function, with magic
methods defined on it.

https://github.com/django/django/blob/3.2.6/django/utils/functional.py#L84-L191.

Signed-off-by: Zixuan James Li <[email protected]>

django-stubs/utils/functional.pyi

@@ -20,7 +20,25 @@ class cached_property(Generic[_T]):
     @overload
     def __get__(self, instance: object, cls: Type[Any] = ...) -> _T: ...
 
-class Promise: ...
+# Promise is only subclassed by a proxy class defined in the lazy function
+# so it makes sense for it to have all the methods available in that proxy class
+class Promise:
+    def __init__(self, args: Any, kw: Any) -> None: ...
+    def __reduce__(self) -> Tuple[Any, Tuple[Any]]: ...
+    @classmethod
+    def __prepare_class__(cls) -> None: ...
+    @classmethod
+    def __promise__(cls, method_name: str) -> Callable: ...
+    def __text_cast(self) -> str: ...
+    def __bytes_cast(self) -> bytes: ...
+    def __bytes_cast_encoded(self) -> bytes: ...
+    def __cast(self) -> Union[bytes, str]: ...
+    def __lt__(self, other: Any) -> bool: ...
+    def __hash__(self) -> int: ...
+    def __mod__(self, rhs: Any) -> str: ...
+    def __add__(self, other: Any) -> str: ...
+    def __radd__(self, other: Any) -> str: ...
+    def __deepcopy__(self, memo: Any): ...
 
 _C = TypeVar("_C", bound=Callable)