Add TypedDict to typing_extensions (#2940)

Since there is a PEP for `TypedDict` (PEP 589), it is being added to `typing_extension`, see https://github.com/python/typing/pull/628. This PR essentially copies the definition from `mypy_extensions`.

Corresponding mypy PR https://github.com/python/mypy/pull/6744 can be merged independently of this PR.

third_party/2and3/typing_extensions.pyi

@@ -1,3 +1,4 @@
+import abc
 import sys
 from typing import Callable
 from typing import ClassVar as ClassVar
@@ -11,8 +12,9 @@ from typing import overload as overload
 from typing import Text as Text
 from typing import Type as Type
 from typing import TYPE_CHECKING as TYPE_CHECKING
-from typing import TypeVar, Any
+from typing import TypeVar, Any, Mapping, ItemsView, KeysView, ValuesView, Dict, Type
 
+_T = TypeVar('_T')
 _F = TypeVar('_F', bound=Callable[..., Any])
 _TC = TypeVar('_TC', bound=Type[object])
 class _SpecialForm:
@@ -23,6 +25,25 @@ Final: _SpecialForm = ...
 def final(f: _F) -> _F: ...
 Literal: _SpecialForm = ...
 
+# Internal mypy fallback type for all typed dicts (does not exist at runtime)
+class _TypedDict(Mapping[str, object], metaclass=abc.ABCMeta):
+    def copy(self: _T) -> _T: ...
+    # Using NoReturn so that only calls using mypy plugin hook that specialize the signature
+    # can go through.
+    def setdefault(self, k: NoReturn, default: object) -> object: ...
+    # Mypy plugin hook for 'pop' expects that 'default' has a type variable type.
+    def pop(self, k: NoReturn, default: _T = ...) -> object: ...
+    def update(self: _T, __m: _T) -> None: ...
+    if sys.version_info < (3, 0):
+        def has_key(self, k: str) -> bool: ...
+        def viewitems(self) -> ItemsView[str, object]: ...
+        def viewkeys(self) -> KeysView[str]: ...
+        def viewvalues(self) -> ValuesView[object]: ...
+    def __delitem__(self, k: NoReturn) -> None: ...
+
+# TypedDict is a (non-subscriptable) special form.
+TypedDict: object = ...
+
 if sys.version_info >= (3, 3):
     from typing import ChainMap as ChainMap