check-code: allow skipping hasattr check in py3-only code

hasattr is safe in Python 3, and in an upcoming patch we can't use
util.safehasattr.

contrib/check-code.py

@@ -300,7 +300,8 b' pypats = ['
     (r'^\s*(while|if) [01]:',
      "use True/False for constant Boolean expression"),
     (r'(?:(?<!def)\s+|\()hasattr\(',
-     'hasattr(foo, bar) is broken, use util.safehasattr(foo, bar) instead'),
+     'hasattr(foo, bar) is broken on py2, use util.safehasattr(foo, bar) '
+     'instead', r'#.*hasattr-py3-only'),
     (r'opener\([^)]*\).read\(',
      "use opener.read() instead"),
     (r'opener\([^)]*\).write\(',