docutils: Use ClassVar for Directive class variables (#11550)

These are intended to be set as class variables, in subclasses of Directive, rather
than instance variables.

See also:
- https://docutils.sourceforge.io/docs/howto/rst-directives.html#the-directive-class
- https://docutils.sourceforge.io/docs/howto/rst-directives.html#admonitions

stubs/docutils/docutils/parsers/rst/__init__.pyi

@@ -24,11 +24,11 @@ class DirectiveError(Exception):
     def __init__(self, level: int, message: str) -> None: ...
 
 class Directive:
-    required_arguments: int
-    optional_arguments: int
-    final_argument_whitespace: bool
-    option_spec: dict[str, Callable[[str], Any]] | None
-    has_content: bool
+    required_arguments: ClassVar[int]
+    optional_arguments: ClassVar[int]
+    final_argument_whitespace: ClassVar[bool]
+    option_spec: ClassVar[dict[str, Callable[[str], Any]] | None]
+    has_content: ClassVar[bool]
     name: str
     arguments: list[str]
     options: dict[str, Any]