Remove __index__ from unnecessary-dunder-call check

doc/whatsnew/fragments/6795.bugfix

@@ -0,0 +1,3 @@
+Remove ``__index__`` dunder method call from ``unnecessary-dunder-call`` check.
+
+Closes #6795

pylint/checkers/dunder_methods.py

@@ -100,7 +100,6 @@
         "__complex__": "Use complex built-in function",
         "__int__": "Use int built-in function",
         "__float__": "Use float built-in function",
-        "__index__": "Use index method",
         "__round__": "Use round built-in function",
         "__trunc__": "Use math.trunc function",
         "__floor__": "Use math.floor function",
@@ -125,7 +124,8 @@ class DunderCallChecker(BaseChecker):
     We exclude __new__, __subclasses__, __init_subclass__, __set_name__,
     __class_getitem__, __missing__, __exit__, __await__,
     __aexit__, __getnewargs_ex__, __getnewargs__, __getstate__,
-    __setstate__, __reduce__, __reduce_ex__
+    __setstate__, __reduce__, __reduce_ex__,
+    and __index__ (see https://github.com/PyCQA/pylint/issues/6795)
     since these either have no alternative method of being called or
     have a genuine use case for being called manually.
 

tests/functional/u/unnecessary/unnecessary_dunder_call.py

@@ -122,3 +122,9 @@ def get_first_subclass(cls):
 # since we can't apply alternate operators/functions here.
 a = [1, 2, 3]
 assert super(type(a), a).__str__() == "[1, 2, 3]"
+
+class MyString(str):
+    """Custom str implementation"""
+    def rjust(self, width, fillchar= ' '):
+        """Acceptable call to __index__"""
+        width = width.__index__()