pytables.open prints to stdout when attempting to open a read-only HDF5 file
https://github.com/pydata/pandas/blob/master/pandas/io/pytables.py#L539

```
    try:
        self._handle = tables.open_file(self._path, self._mode, **kwargs)
    except (IOError) as e:  # pragma: no cover
        if 'can not be written' in str(e):
            print('Opening %s in read-only mode' % self._path)
            self._handle = tables.open_file(self._path, 'r', **kwargs)
        else:
            raise
```

The above code prints to stdout when attempting to read from a readonly file. This is a problem for programs which use stdout for their own output. It should either print to sys.stderr or whatever log the rest of the library uses.
