Remove outdated Python 2.5 mention from special_methods.rst (GH-4538)

docs/src/userguide/special_methods.rst

@@ -379,7 +379,7 @@ https://docs.python.org/3/reference/datamodel.html#emulating-numeric-types
 +-----------------------+---------------------------------------+-------------+-----------------------------------------------------+
 | __hex__ 	        | self 	                                | object      | Convert to hexadecimal                              |
 +-----------------------+---------------------------------------+-------------+-----------------------------------------------------+
-| __index__ (2.5+ only)	| self	                                | object      | Convert to sequence index                           |
+| __index__            	| self	                                | object      | Convert to sequence index                           |
 +-----------------------+---------------------------------------+-------------+-----------------------------------------------------+
 
 In-place arithmetic operators