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/ssl.pyi

@@ -207,6 +207,8 @@ class SSLSocket(socket.socket):
     def unwrap(self) -> socket.socket: ...
     def version(self) -> Optional[str]: ...
     def pending(self) -> int: ...
+    if sys.version_info >= (3, 8):
+        def verify_client_post_handshake(self) -> None: ...
 
 if sys.version_info >= (3, 7):
     class TLSVersion(enum.IntEnum):
@@ -221,6 +223,8 @@ if sys.version_info >= (3, 7):
 class SSLContext:
     check_hostname: bool
     options: int
+    if sys.version_info >= (3, 8):
+        post_handshake_auth: bool
     @property
     def protocol(self) -> int: ...
     verify_flags: int
@@ -285,6 +289,8 @@ if sys.version_info >= (3, 5):
         def do_handshake(self) -> None: ...
         def unwrap(self) -> None: ...
         def get_channel_binding(self, cb_type: str = ...) -> Optional[bytes]: ...
+        if sys.version_info >= (3, 8):
+            def verify_client_post_handshake(self) -> None: ...
 
     class MemoryBIO:
         pending: int