doc: patch Sphinx to detect our @final for marking classes as final

Thanks to Dominic Davis-Foster for code & assistance.

changelog/7780.doc.rst

@@ -0,0 +1 @@
+Classes which should not be inherited from are now marked ``final class`` in the API reference.

doc/en/conf.py

@@ -15,9 +15,10 @@
 #
 # The full version, including alpha/beta/rc tags.
 # The short X.Y version.
+import ast
 import os
 import sys
-from typing import TYPE_CHECKING
+from typing import List
 
 from _pytest import __version__ as version
 
@@ -398,3 +399,22 @@ def setup(app: "sphinx.application.Sphinx") -> None:
     )
 
     configure_logging(app)
+
+    # Make Sphinx mark classes with "final" when decorated with @final.
+    # We need this because we import final from pytest._compat, not from
+    # typing (for Python < 3.8 compat), so Sphinx doesn't detect it.
+    # To keep things simple we accept any `@final` decorator.
+    # Ref: https://github.com/pytest-dev/pytest/pull/7780
+    import sphinx.pycode.ast
+    import sphinx.pycode.parser
+
+    original_is_final = sphinx.pycode.parser.VariableCommentPicker.is_final
+
+    def patched_is_final(self, decorators: List[ast.expr]) -> bool:
+        if original_is_final(self, decorators):
+            return True
+        return any(
+            sphinx.pycode.ast.unparse(decorator) == "final" for decorator in decorators
+        )
+
+    sphinx.pycode.parser.VariableCommentPicker.is_final = patched_is_final