Minor fixes to improve django-stubs (#695)

* Use `Sequence` instead of `Iterable` for `send_messages`.

According to the documentation
(https://docs.djangoproject.com/en/3.2/topics/email/#email-backends),
`email_messages` is a list. Using `Iterable` will make it hard for
subclasses to implement this method utilizing functions like `__len__`.
While this still allows subclasses to accept `Iterable`.

* Fix function signature of `authenticate` of `BaseBackend`.

1. BaseBackend no longer requires the username and password argument.

They were removed 3 years ago in the commit below when `BaseBackend` is added:
https://github.com/django/django/commit/75337a60509fdfdd321a5caf8e30d57fff6b9518

2. `request` is optional for `authenticate` method.

According to django documentation, the authenticate method does not
necessarily require the request object.

https://docs.djangoproject.com/en/3.2/topics/auth/default/#authenticating-users

* Tighten the type of `streaming_content` to `Iterator[bytes]`.

It is an iterator of a bytestring according to the documentation:
https://docs.djangoproject.com/en/3.2/ref/request-response/#django.http.StreamingHttpResponse.streaming_content

* Fix function signature of `django.contrib.staticfiles.serve`.

Since this `serve` function uses `django.views.static.serve` that
accepts `HttpRequest` as its first argument, it is more reasonable
to type it with `HttpRequest` instead of `WSGIRequest`.

Related:
https://github.com/django/django/blob/main/django/contrib/staticfiles/views.py#L39

django-stubs/contrib/auth/backends.pyi

@@ -10,9 +10,7 @@ _AnyUser = Union[Model, AnonymousUser]
 UserModel: Any
 
 class BaseBackend:
-    def authenticate(
-        self, request: HttpRequest, username: Optional[str] = ..., password: Optional[str] = ..., **kwargs: Any
-    ) -> Optional[AbstractBaseUser]: ...
+    def authenticate(self, request: Optional[HttpRequest], **kwargs: Any) -> Optional[AbstractBaseUser]: ...
     def get_user(self, user_id: int) -> Optional[AbstractBaseUser]: ...
     def get_user_permissions(self, user_obj: _AnyUser, obj: Optional[Model] = ...) -> Set[str]: ...
     def get_group_permissions(self, user_obj: _AnyUser, obj: Optional[Model] = ...) -> Set[str]: ...

django-stubs/contrib/staticfiles/views.pyi

@@ -1,6 +1,6 @@
 from typing import Any
 
-from django.core.handlers.wsgi import WSGIRequest
+from django.http.request import HttpRequest
 from django.http.response import FileResponse
 
-def serve(request: WSGIRequest, path: str, insecure: bool = ..., **kwargs: Any) -> FileResponse: ...
+def serve(request: HttpRequest, path: str, insecure: bool = ..., **kwargs: Any) -> FileResponse: ...

django-stubs/core/mail/backends/base.pyi

@@ -1,5 +1,5 @@
 import types
-from typing import Any, Iterable, Optional, Type, TypeVar
+from typing import Any, Optional, Sequence, Type, TypeVar
 
 from django.core.mail.message import EmailMessage
 
@@ -13,4 +13,4 @@ class BaseEmailBackend:
     def __exit__(
         self, exc_type: Type[BaseException], exc_value: BaseException, traceback: types.TracebackType
     ) -> None: ...
-    def send_messages(self, email_messages: Iterable[EmailMessage]) -> int: ...
+    def send_messages(self, email_messages: Sequence[EmailMessage]) -> int: ...

django-stubs/http/response.pyi

@@ -101,8 +101,8 @@ class HttpResponse(HttpResponseBase):
 
 class StreamingHttpResponse(HttpResponseBase):
     content: Any
-    streaming_content: Iterator[Any]
-    def __init__(self, streaming_content: Iterable[Any] = ..., *args: Any, **kwargs: Any) -> None: ...
+    streaming_content: Iterator[bytes]
+    def __init__(self, streaming_content: Iterable[bytes] = ..., *args: Any, **kwargs: Any) -> None: ...
     def getvalue(self) -> bytes: ...
 
 class FileResponse(StreamingHttpResponse):