fix intern() test in Py3

tests/run/intern_T431.pyx

 __doc__ = u"""
 >>> s == s_interned
 True
+>>> s == s_interned_dynamic
+True
+>>> s == 'abc' == s_interned == s_interned_dynamic
+True
+"""
+
+import sys
+if sys.version_info[0] < 3:
+    __doc__ += u"""
 >>> intern(s) is s_interned
 True
 >>> intern('abc') is s_interned