DEPS: Lazy import defusedxml only when necessary (#12362)

The `defusedxml` dependency was added to `sphinx.testing` in sphinx 7.3,
but it is not part of the core sphinx dependencies, and not strictly necessary for the running of the pytest plugin.
This commit moves the import into the only function which uses it,
so that the pytest plugins can be used again without this dependency.

sphinx/testing/util.py

@@ -11,7 +11,6 @@
 from types import MappingProxyType
 from typing import TYPE_CHECKING
 
-from defusedxml.ElementTree import parse as xml_parse
 from docutils import nodes
 from docutils.parsers.rst import directives, roles
 
@@ -73,6 +72,8 @@ def assert_node(node: Node, cls: Any = None, xpath: str = "", **kwargs: Any) ->
 # keep this to restrict the API usage and to have a correct return type
 def etree_parse(path: str | os.PathLike[str]) -> ElementTree:
     """Parse a file into a (safe) XML element tree."""
+    from defusedxml.ElementTree import parse as xml_parse
+
     return xml_parse(path)