Return Promise for lazy functions.

Django uses a proxy method for lazy translation functions that actually
returns instances of `Promise` instead of `str`.

Fixes #688.

django-stubs/utils/translation/__init__.pyi

@@ -4,6 +4,7 @@ from contextlib import ContextDecorator
 from typing import Any, Callable, Optional, Type, Union
 
 from django.http.request import HttpRequest
+from django.utils.functional import Promise
 
 LANGUAGE_SESSION_KEY: str
 
@@ -33,14 +34,14 @@ def ngettext(singular: str, plural: str, number: float) -> str: ...
 def ungettext(singular: str, plural: str, number: float) -> str: ...
 def pgettext(context: str, message: str) -> str: ...
 def npgettext(context: str, singular: str, plural: str, number: int) -> str: ...
+def gettext_lazy(message: str) -> Promise: ...
+def pgettext_lazy(context: str, message: str) -> Promise: ...
+def ngettext_lazy(singular: str, plural: str, number: Union[int, str, None]) -> Promise: ...
 
-gettext_lazy = gettext
-pgettext_lazy = pgettext
+ugettext_lazy = gettext_lazy
+ungettext_lazy = ngettext_lazy
 
-def ugettext_lazy(message: str) -> str: ...
-def ngettext_lazy(singular: str, plural: str, number: Union[int, str, None] = ...) -> str: ...
-def ungettext_lazy(singular: str, plural: str, number: Union[int, str, None] = ...) -> str: ...
-def npgettext_lazy(context: str, singular: str, plural: str, number: Union[int, str, None] = ...) -> str: ...
+def npgettext_lazy(context: str, singular: str, plural: str, number: Union[int, str, None]) -> Promise: ...
 def activate(language: str) -> None: ...
 def deactivate() -> None: ...