Py3: Correct use of string instead of bytes

[kushal-kumaran]

Fixes this traceback:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pyPdf/pdf.py", line 861, in decrypt
    return self._decrypt(password)
  File "pyPdf/pdf.py", line 866, in _decrypt
    encrypt = self.trailer['/Encrypt'].getObject()
  File "pyPdf/generic.py", line 476, in __getitem__
    return dict.__getitem__(self, key).getObject()
  File "pyPdf/generic.py", line 165, in getObject
    return self.pdf.getObject(self).getObject()
  File "pyPdf/pdf.py", line 616, in getObject
    retval = readObject(self.stream, self)
  File "pyPdf/generic.py", line 66, in readObject
    return DictionaryObject.readFromStream(stream, pdf)
  File "pyPdf/generic.py", line 527, in readFromStream
    value = readObject(stream, pdf)
  File "pyPdf/generic.py", line 51, in readObject
    return readStringFromStream(stream)
  File "pyPdf/generic.py", line 337, in readStringFromStream
    txt.extend(tok)
TypeError: an integer is required