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/2and3/socket.pyi

@@ -585,6 +585,16 @@ class socket:
 def create_connection(address: Tuple[Optional[str], int],
                       timeout: Optional[float] = ...,
                       source_address: Tuple[Union[bytearray, bytes, Text], int] = ...) -> socket: ...
+if sys.version_info >= (3, 8):
+    def has_dualstack_ipv6() -> bool: ...
+    def create_server(
+        address: Tuple[str, int],
+        *,
+        family: AddressFamily = ...,
+        backlog: Optional[int] = ...,
+        reuse_port: bool = ...,
+        dualstack_ipv6: bool = ...,
+    ) -> socket: ...
 
 # the 5th tuple item is an address
 # TODO the "Tuple[Any, ...]" should be "Union[Tuple[str, int], Tuple[str, int, int, int]]" but that triggers