add missing files throughout the codebase (#102)

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

@@ -0,0 +1,16 @@
+import types
+from typing import Any, TypeVar, Type, Iterable
+
+from django.core.mail.message import EmailMessage
+
+_T = TypeVar("_T", bound="BaseEmailBackend")
+
+class BaseEmailBackend:
+    def __init__(self, fail_silently: bool = ..., **kwargs: Any) -> None: ...
+    def open(self) -> bool: ...
+    def close(self) -> None: ...
+    def __enter__(self: _T) -> _T: ...
+    def __exit__(
+        self, exc_type: Type[BaseException], exc_value: BaseException, traceback: types.TracebackType
+    ) -> None: ...
+    def send_messages(self, email_messages: Iterable[EmailMessage]) -> int: ...