Recognize 'U' as a valid read-only mode for open()

--HG--
branch : setuptools-0.6
extra : convert_revision : svn%3A6015fed2-1504-0410-9fe1-9d1591cc4771/sandbox/branches/setuptools-0.6%4045629

setuptools/sandbox.py

@@ -158,7 +158,7 @@ class DirectorySandbox(AbstractSandbox):
         raise SandboxViolation(operation, args, kw)
 
     def _open(self, path, mode='r', *args, **kw):
-        if mode not in ('r', 'rt', 'rb', 'rU') and not self._ok(path):
+        if mode not in ('r', 'rt', 'rb', 'rU', 'U') and not self._ok(path):
             self._violation("open", path, mode, *args, **kw)
         return _open(path,mode,*args,**kw)