Complete type stubs for xml.dom.minidom (python#6886)

This patch completes all the type stubs for xml.dom.minidom.  I did
not look at the rest of the xml.dom directory except as required to
make the typing of minidom complete, so it probably doesn’t completely
resolve python#6886.

xml.dom.minidom is very poorly documented—its authors seem to have
assumed that one could refer to the W3C DOM spec.  But at the same
time the implementation diverges significantly from the spec, so I
do not have complete confidence in all the type annotations.  Still,
it should be an improvement on what’s there now.

The following commands now pass with no errors when executed from the
top of the typeshed tree:

```sh
mypy --custom-typeshed-dir . --strict --warn-incomplete-stub \
     -m xml.dom.minidom
stubtest --custom-typeshed-dir . xml.dom.minidom
```

**However**, attempting to do the same for xml.dom.minicompat produces
the following error:

```sh
mypy --custom-typeshed-dir . --strict --warn-incomplete-stub \
     -m xml.dom.minicompat
stdlib/xml/dom/minicompat.pyi:11: error: All bases of a protocol must be protocols  [misc]
```

I don’t know how to fix this, because in the implementation,
EmptyNodeList is not a subclass of NodeList, therefore if I don’t make
NodeList a protocol, then xml.dom.minidom.Childless fails stubtest.
This patch probably should not land until this is resolved.  Any
advice you could give would be welcome.

Author: zackw

stdlib/xml/dom/__init__.pyi

@@ -1,4 +1,4 @@
-from typing import Any
+from typing import Any, Protocol
 
 from .domreg import getDOMImplementation as getDOMImplementation, registerDOMImplementation as registerDOMImplementation
 
@@ -56,12 +56,14 @@ class NamespaceErr(DOMException): ...
 class InvalidAccessErr(DOMException): ...
 class ValidationErr(DOMException): ...
 
-class UserDataHandler:
+class UserDataHandler(Protocol):
     NODE_CLONED: int
     NODE_IMPORTED: int
     NODE_DELETED: int
     NODE_RENAMED: int
 
+    def handle(self, operation: int, key: str, data: object, src: Node, dst: Node) -> None: ...
+
 XML_NAMESPACE: str
 XMLNS_NAMESPACE: str
 XHTML_NAMESPACE: str

stdlib/xml/dom/minicompat.pyi

@@ -1,14 +1,14 @@
-from collections.abc import Iterable
-from typing import Any, TypeVar
+from collections.abc import Iterable, Sequence
+from typing import Any, Protocol, TypeVar
 from typing_extensions import Literal
 
 __all__ = ["NodeList", "EmptyNodeList", "StringTypes", "defproperty"]
 
-_T = TypeVar("_T")
+_T = TypeVar("_T", covariant=True)
 
 StringTypes: tuple[type[str]]
 
-class NodeList(list[_T]):
+class NodeList(Sequence[_T], Protocol):
     @property
     def length(self) -> int: ...
     def item(self, index: int) -> _T | None: ...