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/xml/etree/ElementTree.pyi

@@ -1,6 +1,26 @@
 # Stubs for xml.etree.ElementTree
 
-from typing import Any, Callable, Dict, Generator, IO, ItemsView, Iterable, Iterator, KeysView, List, MutableSequence, Optional, overload, Sequence, Text, Tuple, TypeVar, Union
+from typing import (
+    Any,
+    Callable,
+    Dict,
+    Generator,
+    IO,
+    ItemsView,
+    Iterable,
+    Iterator,
+    KeysView,
+    List,
+    MutableSequence,
+    Optional,
+    Protocol,
+    Sequence,
+    Text,
+    Tuple,
+    TypeVar,
+    Union,
+    overload,
+)
 import io
 import sys
 
@@ -43,6 +63,41 @@ else:
     # _fixtext function in the source). Client code knows best:
     _str_result_type = Any
 
+_file_or_filename = Union[str, bytes, int, IO[Any]]
+
+if sys.version_info >= (3, 8):
+    class _Writeable(Protocol):
+        def write(self, __s: str) -> Any: ...
+
+    @overload
+    def canonicalize(
+        xml_data: Optional[_parser_input_type] = ...,
+        *,
+        out: None = ...,
+        from_file: Optional[_file_or_filename] = ...,
+        with_comments: bool = ...,
+        strip_text: bool = ...,
+        rewrite_prefixes: bool = ...,
+        qname_aware_tags: Optional[Iterable[str]] = ...,
+        qname_aware_attrs: Optional[Iterable[str]] = ...,
+        exclude_attrs: Optional[Iterable[str]] = ...,
+        exclude_tags: Optional[Iterable[str]] = ...,
+    ) -> str: ...
+    @overload
+    def canonicalize(
+        xml_data: Optional[_parser_input_type] = ...,
+        *,
+        out: _Writeable,
+        from_file: Optional[_file_or_filename] = ...,
+        with_comments: bool = ...,
+        strip_text: bool = ...,
+        rewrite_prefixes: bool = ...,
+        qname_aware_tags: Optional[Iterable[str]] = ...,
+        qname_aware_attrs: Optional[Iterable[str]] = ...,
+        exclude_attrs: Optional[Iterable[str]] = ...,
+        exclude_tags: Optional[Iterable[str]] = ...,
+    ) -> None: ...
+
 class Element(MutableSequence[Element]):
     tag: _str_result_type
     attrib: Dict[_str_result_type, _str_result_type]
@@ -100,9 +155,6 @@ class QName:
     text: str
     def __init__(self, text_or_uri: _str_argument_type, tag: Optional[_str_argument_type] = ...) -> None: ...
 
-
-_file_or_filename = Union[str, bytes, int, IO[Any]]
-
 class ElementTree:
     def __init__(self, element: Optional[Element] = ..., file: Optional[_file_or_filename] = ...) -> None: ...
     def getroot(self) -> Element: ...
@@ -176,6 +228,22 @@ class TreeBuilder:
     def start(self, tag: _parser_input_type, attrs: Dict[_parser_input_type, _parser_input_type]) -> Element: ...
     def end(self, tag: _parser_input_type) -> Element: ...
 
+if sys.version_info >= (3, 8):
+    class C14NWriterTarget:
+        def __init__(
+            self,
+            write: Callable[[str], Any],
+            *,
+            with_comments: bool = ...,
+            strip_text: bool = ...,
+            rewrite_prefixes: bool = ...,
+            qname_aware_tags: Optional[Iterable[str]] = ...,
+            qname_aware_attrs: Optional[Iterable[str]] = ...,
+            exclude_attrs: Optional[Iterable[str]] = ...,
+            exclude_tags: Optional[Iterable[str]] = ...,
+        ) -> None: ...
+
+
 class XMLParser:
     parser: Any
     target: TreeBuilder