Add a few missing stdlib modules (#4550)

stdlib/2and3/xml/dom/__init__.pyi

@@ -1,5 +1,7 @@
 from typing import Any
 
+from .domreg import getDOMImplementation as getDOMImplementation, registerDOMImplementation as registerDOMImplementation
+
 class Node:
     ELEMENT_NODE: int
     ATTRIBUTE_NODE: int

stdlib/2and3/xml/dom/domreg.pyi

@@ -0,0 +1,10 @@
+from _typeshed.xml import DOMImplementation
+from typing import Any, Callable, Dict, Iterable, Optional, Tuple, Union
+
+well_known_implementations: Dict[str, str]
+registered: Dict[str, Callable[[], DOMImplementation]]
+
+def registerDOMImplementation(name: str, factory: Callable[[], DOMImplementation]) -> None: ...
+def getDOMImplementation(
+    name: Optional[str] = ..., features: Union[str, Iterable[Tuple[str, Optional[str]]]] = ...
+) -> DOMImplementation: ...