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/threading.pyi

@@ -29,6 +29,9 @@ def enumerate() -> List[Thread]: ...
 if sys.version_info >= (3, 4):
     def main_thread() -> Thread: ...
 
+if sys.version_info >= (3, 8):
+    from _thread import get_native_id as get_native_id
+
 def settrace(func: _TF) -> None: ...
 def setprofile(func: Optional[_PF]) -> None: ...
 def stack_size(size: int = ...) -> int: ...
@@ -67,6 +70,9 @@ class Thread:
     def join(self, timeout: Optional[float] = ...) -> None: ...
     def getName(self) -> str: ...
     def setName(self, name: str) -> None: ...
+    if sys.version_info >= (3, 8):
+        @property
+        def native_id(self) -> Optional[int]: ...  # only available on some platforms
     def is_alive(self) -> bool: ...
     def isAlive(self) -> bool: ...
     def isDaemon(self) -> bool: ...
@@ -160,6 +166,9 @@ class Event:
     def clear(self) -> None: ...
     def wait(self, timeout: Optional[float] = ...) -> bool: ...
 
+if sys.version_info >= (3, 8):
+    from _thread import _ExceptHookArgs as ExceptHookArgs, ExceptHookArgs as _ExceptHookArgs  # don't ask
+    excepthook: Callable[[_ExceptHookArgs], Any]
 
 class Timer(Thread):
     if sys.version_info >= (3,):