Use classmethod for a class method

Author: AA-Turner

Diff for: CHANGES.rst

@@ -4,6 +4,8 @@ Release 8.0.2 (in development)
 Bugs fixed
 ----------
 
+* Fix the ``pygments.Formatter.__class_getitem__`` patch.
+  Patch by Adam Turner.
 
 Release 8.0.1 (released Jul 30, 2024)
 =====================================

Diff for: sphinx/highlighting.py

@@ -34,7 +34,7 @@
 if tuple(map(int, pygments.__version__.split('.')))[:2] < (2, 18):
     from pygments.formatter import Formatter  # NoQA: F811
 
-    Formatter.__class_getitem__ = lambda cls, name: cls  # type: ignore[attr-defined]
+    Formatter.__class_getitem__ = classmethod(lambda cls, name: cls)  # type: ignore[attr-defined]
 
 logger = logging.getLogger(__name__)
 

Diff for: tests/test_highlighting.py

@@ -11,7 +11,7 @@
 
 if tuple(map(int, pygments.__version__.split('.')))[:2] < (2, 18):
     from pygments.formatter import Formatter
-    Formatter.__class_getitem__ = lambda cls, name: cls  # type: ignore[attr-defined]
+    Formatter.__class_getitem__ = classmethod(lambda cls, name: cls)  # type: ignore[attr-defined]
 
 
 class MyLexer(RegexLexer):