change istestfunction to callable() (#8374)

Author: symonk

src/_pytest/python.py

@@ -384,10 +384,7 @@ def istestfunction(self, obj: object, name: str) -> bool:
             if isinstance(obj, staticmethod):
                 # staticmethods need to be unwrapped.
                 obj = safe_getattr(obj, "__func__", False)
-            return (
-                safe_getattr(obj, "__call__", False)
-                and fixtures.getfixturemarker(obj) is None
-            )
+            return callable(obj) and fixtures.getfixturemarker(obj) is None
         else:
             return False