Fix instantiating a xml.etree.ElementTree.Element (#1930)

The methods removed by https://github.com/python/typeshed/pull/1816
are abstract in `MutableSequence` and therefore must be specified on `Element`.

stdlib/2and3/xml/etree/ElementTree.pyi

@@ -1,6 +1,6 @@
 # Stubs for xml.etree.ElementTree
 
-from typing import Any, Callable, Dict, Generator, IO, ItemsView, Iterable, Iterator, KeysView, List, MutableSequence, Optional, Sequence, Text, Tuple, TypeVar, Union
+from typing import Any, Callable, Dict, Generator, IO, ItemsView, Iterable, Iterator, KeysView, List, MutableSequence, Optional, overload, Sequence, Text, Tuple, TypeVar, Union
 import io
 import sys
 
@@ -70,6 +70,17 @@ class Element(MutableSequence['Element']):
     def remove(self, subelement: 'Element') -> None: ...
     def set(self, key: _str_argument_type, value: _str_argument_type) -> None: ...
     def __bool__(self) -> bool: ...
+    def __delitem__(self, i: Union[int, slice]) -> None: ...
+    @overload
+    def __getitem__(self, i: int) -> 'Element': ...
+    @overload
+    def __getitem__(self, s: slice) -> Sequence['Element']: ...
+    def __len__(self) -> int: ...
+    @overload
+    def __setitem__(self, i: int, o: 'Element') -> None: ...
+    @overload
+    def __setitem__(self, s: slice, o: Iterable['Element']) -> None: ...
+
 
 def SubElement(parent: Element, tag: _str_argument_type, attrib: Dict[_str_argument_type, _str_argument_type]=..., **extra: _str_argument_type) -> Element: ...
 def Comment(text: _str_argument_type=...) -> Element: ...