Improve accuracy of six byte index methods (#9117)

stubs/six/@tests/stubtest_allowlist.txt

@@ -8,8 +8,6 @@ six.create_bound_method.__defaults__
 six.moves.*
 
 # Implemented using "operator" functions in the implementation
-six.byte2int
-six.indexbytes
 six.get_function_closure
 six.get_function_code
 six.get_function_defaults
@@ -19,6 +17,8 @@ six.get_method_self
 six.viewitems
 six.viewkeys
 six.viewvalues
+# Should be `operator.itemgetter[int]`. But a bug in mypy prevents using TypeVar in itemgetter__call__
+six.byte2int
 
 # Unclear problems
 six.callable