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/os/__init__.pyi

@@ -210,6 +210,8 @@ class stat_result:
     st_atime_ns: int  # time of most recent access, in nanoseconds
     st_mtime_ns: int  # time of most recent content modification in nanoseconds
     st_ctime_ns: int  # platform dependent (time of most recent metadata change on Unix, or the time of creation on Windows) in nanoseconds
+    if sys.version_info >= (3, 8) and sys.platform == "win32":
+        st_reparse_tag: int
 
     def __getitem__(self, i: int) -> int: ...
 
@@ -631,3 +633,31 @@ else:
 
 if sys.version_info >= (3, 7):
     def register_at_fork(func: Callable[..., object], when: str) -> None: ...
+
+if sys.version_info >= (3, 8):
+    if sys.platform == "win32":
+        class _AddedDllDirectory:
+            path: Optional[str]
+            def close(self) -> None: ...
+            def __enter__(self: _T) -> _T: ...
+            def __exit__(self, *args: Any) -> None: ...
+        def add_dll_directory(path: str) -> _AddedDllDirectory: ...
+    if sys.platform == "linux":
+        MFD_CLOEXEC: int
+        MFD_ALLOW_SEALING: int
+        MFD_HUGETLB: int
+        MFD_HUGE_SHIFT: int
+        MFD_HUGE_MASK: int
+        MFD_HUGE_64KB: int
+        MFD_HUGE_512KB: int
+        MFD_HUGE_1MB: int
+        MFD_HUGE_2MB: int
+        MFD_HUGE_8MB: int
+        MFD_HUGE_16MB: int
+        MFD_HUGE_32MB: int
+        MFD_HUGE_256MB: int
+        MFD_HUGE_512MB: int
+        MFD_HUGE_1GB: int
+        MFD_HUGE_2GB: int
+        MFD_HUGE_16GB: int
+        def memfd_create(name: str, flags: int = ...) -> int: ...