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/sys.pyi

@@ -54,8 +54,6 @@ stderr: TextIO
 __stdin__: TextIO
 __stdout__: TextIO
 __stderr__: TextIO
-# deprecated and removed in Python 3.3:
-subversion: Tuple[str, str, str]
 tracebacklimit: int
 version: str
 api_version: int
@@ -198,3 +196,13 @@ def setswitchinterval(interval: float) -> None: ...
 def settscdump(on_flag: bool) -> None: ...
 
 def gettotalrefcount() -> int: ...  # Debug builds only
+
+if sys.version_info >= (3, 8):
+    # not exported by sys
+    class UnraisableHookArgs:
+        exc_type: Type[BaseException]
+        exc_value: Optional[BaseException]
+        exc_traceback: Optional[TracebackType]
+        err_msg: Optional[str]
+        object: Optional[object]
+    unraisablehook: Callable[[UnraisableHookArgs], Any]