Update protobuf ExtensionDict (#4619)

third_party/2and3/google/protobuf/internal/extension_dict.pyi

@@ -0,0 +1,22 @@
+from typing import Any, Generic, Iterator, TypeVar
+
+from google.protobuf.descriptor import FieldDescriptor
+from google.protobuf.message import Message
+
+_ContainerMessageT = TypeVar("_ContainerMessageT", bound=Message)
+_ExtenderMessageT = TypeVar("_ExtenderMessageT", bound=Message)
+
+class _ExtensionFieldDescriptor(FieldDescriptor, Generic[_ContainerMessageT, _ExtenderMessageT]): ...
+
+class _ExtensionDict(Generic[_ContainerMessageT]):
+    def __init__(self, extended_message: _ContainerMessageT) -> None: ...
+    def __getitem__(
+        self, extension_handle: _ExtensionFieldDescriptor[_ContainerMessageT, _ExtenderMessageT]
+    ) -> _ExtenderMessageT: ...
+    def __setitem__(
+        self, extension_handle: _ExtensionFieldDescriptor[_ContainerMessageT, _ExtenderMessageT], value: _ExtenderMessageT
+    ) -> None: ...
+    def __delitem__(self, extension_handle: _ExtensionFieldDescriptor[_ContainerMessageT, _ExtenderMessageT]) -> None: ...
+    def __contains__(self, extension_handle: _ExtensionFieldDescriptor[_ContainerMessageT, _ExtenderMessageT]) -> bool: ...
+    def __iter__(self) -> Iterator[_ExtensionFieldDescriptor[_ContainerMessageT, Any]]: ...
+    def __len__(self) -> int: ...

third_party/2and3/google/protobuf/message.pyi

@@ -1,16 +1,13 @@
 import sys
 from typing import Any, ByteString, Sequence, Tuple, Type, TypeVar, Union
 
-from .descriptor import Descriptor, DescriptorBase, FieldDescriptor
+from .descriptor import Descriptor, FieldDescriptor
+from .internal.extension_dict import _ExtensionDict
 
 class Error(Exception): ...
 class DecodeError(Error): ...
 class EncodeError(Error): ...
 
-class _ExtensionDict:
-    def __getitem__(self, extension_handle: DescriptorBase) -> Any: ...
-    def __setitem__(self, extension_handle: DescriptorBase, value: Any) -> None: ...
-
 _M = TypeVar("_M", bound=Message)  # message type (of self)
 
 if sys.version_info < (3,):
@@ -39,7 +36,7 @@ class Message:
     @classmethod
     def FromString(cls: Type[_M], s: _Serialized) -> _M: ...
     @property
-    def Extensions(self) -> _ExtensionDict: ...
+    def Extensions(self: _M) -> _ExtensionDict[_M]: ...
     # Intentionally left out typing on these three methods, because they are
     # stringly typed and it is not useful to call them on a Message directly.
     # We prefer more specific typing on individual subclasses of Message