xml.etree.ElementTree: fix missing None in get(), findtext() return type

Before, this code

    from xml.etree.ElementTree import Element
    reveal_type(Element('foo').findtext('bar'))

would reveal `builtins.str`. This is incorrect because if the path is
not found, a default None is returned.

After, it reveals `Union[builtins.str, None]`.

The solution with `overload`s is the same as used by `Mapping.get` in
`stdlib/3/typing.pyi`.

Author: bluetech

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, overload, 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, overload
 import io
 import sys
 
@@ -53,8 +53,14 @@ class Element(MutableSequence[Element]):
     def extend(self, elements: Iterable[Element]) -> None: ...
     def find(self, path: _str_argument_type, namespaces: Optional[Dict[_str_argument_type, _str_argument_type]] = ...) -> Optional[Element]: ...
     def findall(self, path: _str_argument_type, namespaces: Optional[Dict[_str_argument_type, _str_argument_type]] = ...) -> List[Element]: ...
-    def findtext(self, path: _str_argument_type, default: Optional[_T] = ..., namespaces: Optional[Dict[_str_argument_type, _str_argument_type]] = ...) -> Union[_T, _str_result_type]: ...
-    def get(self, key: _str_argument_type, default: Optional[_T] = ...) -> Union[_str_result_type, _T]: ...
+    @overload
+    def findtext(self, path: _str_argument_type, namespaces: Optional[Dict[_str_argument_type, _str_argument_type]] = ...) -> Optional[_str_result_type]: ...
+    @overload
+    def findtext(self, path: _str_argument_type, default: _T, namespaces: Optional[Dict[_str_argument_type, _str_argument_type]] = ...) -> Union[_T, _str_result_type]: ...
+    @overload
+    def get(self, key: _str_argument_type) -> Optional[_str_result_type]: ...
+    @overload
+    def get(self, key: _str_argument_type, default: _T) -> Union[_str_result_type, _T]: ...
     def getchildren(self) -> List[Element]: ...
     def getiterator(self, tag: Optional[_str_argument_type] = ...) -> List[Element]: ...
     if sys.version_info >= (3, 2):
@@ -102,7 +108,10 @@ class ElementTree:
     def iter(self, tag: Optional[_str_argument_type] = ...) -> Generator[Element, None, None]: ...
     def getiterator(self, tag: Optional[_str_argument_type] = ...) -> List[Element]: ...
     def find(self, path: _str_argument_type, namespaces: Optional[Dict[_str_argument_type, _str_argument_type]] = ...) -> Optional[Element]: ...
-    def findtext(self, path: _str_argument_type, default: Optional[_T] = ..., namespaces: Optional[Dict[_str_argument_type, _str_argument_type]] = ...) -> Union[_T, _str_result_type]: ...
+    @overload
+    def findtext(self, path: _str_argument_type, namespaces: Optional[Dict[_str_argument_type, _str_argument_type]] = ...) -> Optional[_str_result_type]: ...
+    @overload
+    def findtext(self, path: _str_argument_type, default: _T, namespaces: Optional[Dict[_str_argument_type, _str_argument_type]] = ...) -> Union[_T, _str_result_type]: ...
     def findall(self, path: _str_argument_type, namespaces: Optional[Dict[_str_argument_type, _str_argument_type]] = ...) -> List[Element]: ...
     def iterfind(self, path: _str_argument_type, namespaces: Optional[Dict[_str_argument_type, _str_argument_type]] = ...) -> List[Element]: ...
     if sys.version_info >= (3, 4):