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/unicodedata.pyi

@@ -1,4 +1,4 @@
-# Stubs for unicodedata (Python 2.7 and 3.4)
+import sys
 from typing import Any, Text, TypeVar, Union
 
 ucd_3_2_0: UCD
@@ -14,6 +14,8 @@ def decimal(__chr: Text, __default: _default = ...) -> Union[int, _default]: ...
 def decomposition(__chr: Text) -> Text: ...
 def digit(__chr: Text, __default: _default = ...) -> Union[int, _default]: ...
 def east_asian_width(__chr: Text) -> Text: ...
+if sys.version_info >= (3, 8):
+    def is_normalized(__form: str, __unistr: str) -> bool: ...
 def lookup(__name: Union[Text, bytes]) -> Text: ...
 def mirrored(__chr: Text) -> int: ...
 def name(__chr: Text, __default: _default = ...) -> Union[Text, _default]: ...