Relax type of xml.etree.ElementTree.XMLParser target argument (#4538)

The target argument can be an arbitrary object.
If it has certain methods, they are used by XMLParser,
missing methods are ignored.

Ideally, we'd be able to type the potentially missing
methods correctly, but currently the type system is
unable to do so.

Fixes #4537

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

@@ -363,14 +363,14 @@ if sys.version_info >= (3, 8):
 
 class XMLParser:
     parser: Any
-    target: TreeBuilder
+    target: Any
     # TODO-what is entity used for???
     entity: Any
     version: str
     if sys.version_info >= (3, 8):
-        def __init__(self, *, target: Optional[TreeBuilder] = ..., encoding: Optional[str] = ...) -> None: ...
+        def __init__(self, *, target: Any = ..., encoding: Optional[str] = ...) -> None: ...
     else:
-        def __init__(self, html: int = ..., target: Optional[TreeBuilder] = ..., encoding: Optional[str] = ...) -> None: ...
+        def __init__(self, html: int = ..., target: Any = ..., encoding: Optional[str] = ...) -> None: ...
         def doctype(self, __name: str, __pubid: str, __system: str) -> None: ...
-    def close(self) -> Element: ...
+    def close(self) -> Any: ...
     def feed(self, __data: _parser_input_type) -> None: ...