otptoken: support Python 3 for the qr code
    
    When IPA client is using Python 3, there is no str.decode() method
    anymore.
    
    ipa: ERROR: AttributeError: 'str' object has no attribute 'decode'
    Traceback (most recent call last):
     File "/usr/lib/python3.5/site-packages/ipalib/cli.py", line 1345, in run
       sys.exit(api.Backend.cli.run(argv))
     File "/usr/lib/python3.5/site-packages/ipalib/cli.py", line 1110, in run
       rv = cmd.output_for_cli(self.api.Backend.textui, result, *args, **options)
     File "/usr/lib/python3.5/site-packages/ipaclient/plugins/otptoken.py", line 103, in output_for_cli
       qr = self._get_qrcode(output, uri, options['version'])
     File "/usr/lib/python3.5/site-packages/ipaclient/plugins/otptoken.py", line 61, in _get_qrcode
       qr_code = qr_output.getvalue().decode(encoding)
    AttributeError: 'str' object has no attribute 'decode' ipa: ERROR: an internal error has occurred
    
    Fixes https://fedorahosted.org/freeipa/ticket/5938
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>