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/types.pyi

@@ -40,6 +40,8 @@ LambdaType = FunctionType
 class CodeType:
     """Create a code object.  Not for the faint of heart."""
     co_argcount: int
+    if sys.version_info >= (3, 8):
+        co_posonlyargcount: int
     co_kwonlyargcount: int
     co_nlocals: int
     co_stacksize: int
@@ -54,24 +56,45 @@ class CodeType:
     co_lnotab: bytes
     co_freevars: Tuple[str, ...]
     co_cellvars: Tuple[str, ...]
-    def __init__(
-        self,
-        argcount: int,
-        kwonlyargcount: int,
-        nlocals: int,
-        stacksize: int,
-        flags: int,
-        codestring: bytes,
-        constants: Tuple[Any, ...],
-        names: Tuple[str, ...],
-        varnames: Tuple[str, ...],
-        filename: str,
-        name: str,
-        firstlineno: int,
-        lnotab: bytes,
-        freevars: Tuple[str, ...] = ...,
-        cellvars: Tuple[str, ...] = ...,
-    ) -> None: ...
+    if sys.version_info >= (3, 8):
+        def __init__(
+            self,
+            argcount: int,
+            posonlyargcount: int,
+            kwonlyargcount: int,
+            nlocals: int,
+            stacksize: int,
+            flags: int,
+            codestring: bytes,
+            constants: Tuple[Any, ...],
+            names: Tuple[str, ...],
+            varnames: Tuple[str, ...],
+            filename: str,
+            name: str,
+            firstlineno: int,
+            lnotab: bytes,
+            freevars: Tuple[str, ...] = ...,
+            cellvars: Tuple[str, ...] = ...,
+        ) -> None: ...
+    else:
+        def __init__(
+            self,
+            argcount: int,
+            kwonlyargcount: int,
+            nlocals: int,
+            stacksize: int,
+            flags: int,
+            codestring: bytes,
+            constants: Tuple[Any, ...],
+            names: Tuple[str, ...],
+            varnames: Tuple[str, ...],
+            filename: str,
+            name: str,
+            firstlineno: int,
+            lnotab: bytes,
+            freevars: Tuple[str, ...] = ...,
+            cellvars: Tuple[str, ...] = ...,
+        ) -> None: ...
     if sys.version_info >= (3, 8):
         def replace(
             self,