Docs: Update TypedDict import statements

Since Python 3.8, `TypedDict` has been available from the `typing` module. 

As Python 3.8+ is needed to use mypy (https://github.com/python/mypy/blob/master/setup.py#L12), then it's best for the docs to reflect Python 3.8+ usage.

For previous versions, there's already a disclaimer on the page that explains that `typing_extensions` must be used: https://github.com/python/mypy/blob/master/docs/source/typed_dict.rst?plain=1#L102-L110

docs/source/typed_dict.rst

@@ -25,7 +25,7 @@ dictionary value depends on the key:
 
 .. code-block:: python
 
-   from typing_extensions import TypedDict
+   from typing import TypedDict
 
    Movie = TypedDict('Movie', {'name': str, 'year': int})
 
@@ -189,7 +189,7 @@ in Python 3.6 and later:
 
 .. code-block:: python
 
-   from typing_extensions import TypedDict
+   from typing import TypedDict
 
    class Movie(TypedDict):
        name: str