check-code: detect legacy octal syntax

Now that we have mass rewriting all files to use the modern octal
syntax, detect and ban the legacy syntax, which is no longer supported
in Python 3.

contrib/check-code.py

@@ -288,6 +288,7 b' pypats = ['
     (r'\b__bool__\b', "__bool__ should be __nonzero__ in Python 2"),
     (r'os\.path\.join\(.*, *(""|\'\')\)',
      "use pathutil.normasprefix(path) instead of os.path.join(path, '')"),
+    (r'\s0[0-7]+\b', 'legacy octal syntax; use "0o" prefix instead of "0"'),
   ],
   # warnings
   [