Annotations for remaining Python 3.8 additions (#3358)

* Add os.add_dll_directory()
* Add memfd_create() and flags
* Add type annotation to flags
* Add stat_result.st_reparse_tag and flags
* Add ncurses_version
* Add Path.link_to()
* Add Picker.reducer_override()
* Add plistlib.UID
* Add has_dualstack_ipv6() and create_server()
* Add shlex.join()
* Add SSL methods and fields
* Add Python 3.8 statistics functions and classes
* Remove obsolete sys.subversion
* Add sys.unraisablehook
* Add threading.excepthook
* Add get_native_id() and Thread.native_id
* Add Python 3.8 tkinter methods
* Add CLOCK_UPTIME_RAW
* Add SupportsIndex
* Add typing.get_origin() and get_args()
* Add unicodedata.is_normalized
* Add unittest.mock.AsyncMock

Currently this is just an alias for Any like Mock and MagicMock. All of
these classes should probably be sub-classing Any and add their own
methods. See also #3224.

* Add unittest cleanup methods
* Add IsolatedAsyncioTestCase
* Add ElementTree.canonicalize() and C14NWriterTarget
* cProfile.Profile can be used as a context manager
* Add asyncio task name handling
* mmap.flush() now always returns None
* Add posonlyargcount to CodeType

stdlib/3/typing.pyi

@@ -99,6 +99,12 @@ class SupportsBytes(Protocol, metaclass=ABCMeta):
     @abstractmethod
     def __bytes__(self) -> bytes: ...
 
+if sys.version_info >= (3, 8):
+    @runtime_checkable
+    class SupportsIndex(Protocol, metaclass=ABCMeta):
+        @abstractmethod
+        def __index__(self) -> int: ...
+
 @runtime_checkable
 class SupportsAbs(Protocol[_T_co]):
     @abstractmethod
@@ -590,6 +596,9 @@ class Pattern(Generic[AnyStr]):
 def get_type_hints(
     obj: Callable[..., Any], globalns: Optional[Dict[str, Any]] = ..., localns: Optional[Dict[str, Any]] = ...,
 ) -> Dict[str, Any]: ...
+if sys.version_info >= (3, 8):
+    def get_origin(tp: Any) -> Optional[Any]: ...
+    def get_args(tp: Any) -> Tuple[Any, ...]: ...
 
 @overload
 def cast(tp: Type[_T], obj: Any) -> _T: ...