bpo-29262: Add get_origin() and get_args() introspection helpers to typing (GH-13685)

This is an old feature request that appears from time to time. After a year of experimenting with various introspection capabilities in `typing_inspect` on PyPI, I propose to add these two most commonly used functions: `get_origin()` and `get_args()`. These are essentially thin public wrappers around private APIs: `__origin__` and `__args__`.

As discussed in the issue and on the typing tracker, exposing some public helpers instead of `__origin__` and `__args__` directly will give us more flexibility if we will decide to update the internal representation, while still maintaining backwards compatibility.

The implementation is very simple an is essentially a copy from `typing_inspect` with one exception: `ClassVar` was special-cased in `typing_inspect`, but I think this special-casing doesn't really help and only makes things more complicated.

Lib/typing.py

@@ -99,6 +99,8 @@ __all__ = [
     'AnyStr',
     'cast',
     'final',
+    'get_args',
+    'get_origin',
     'get_type_hints',
     'NewType',
     'no_type_check',
@@ -1253,6 +1255,46 @@ def get_type_hints(obj, globalns=None, localns=None):
     return hints
 
 
+def get_origin(tp):
+    """Get the unsubscripted version of a type.
+
+    This supports generic types, Callable, Tuple, Union, Literal, Final and ClassVar.
+    Return None for unsupported types. Examples::
+
+        get_origin(Literal[42]) is Literal
+        get_origin(int) is None
+        get_origin(ClassVar[int]) is ClassVar
+        get_origin(Generic) is Generic
+        get_origin(Generic[T]) is Generic
+        get_origin(Union[T, int]) is Union
+        get_origin(List[Tuple[T, T]][int]) == list
+    """
+    if isinstance(tp, _GenericAlias):
+        return tp.__origin__
+    if tp is Generic:
+        return Generic
+    return None
+
+
+def get_args(tp):
+    """Get type arguments with all substitutions performed.
+
+    For unions, basic simplifications used by Union constructor are performed.
+    Examples::
+        get_args(Dict[str, int]) == (str, int)
+        get_args(int) == ()
+        get_args(Union[int, Union[T, int], str][int]) == (int, str)
+        get_args(Union[int, Tuple[T, int]][str]) == (int, Tuple[str, int])
+        get_args(Callable[[], T][int]) == ([], int)
+    """
+    if isinstance(tp, _GenericAlias):
+        res = tp.__args__
+        if get_origin(tp) is collections.abc.Callable and res[0] is not Ellipsis:
+            res = (list(res[:-1]), res[-1])
+        return res
+    return ()
+
+
 def no_type_check(arg):
     """Decorator to indicate that annotations are not type hints.