In Python 3, urlencode() returns Unicode string; but urlopen() wants
only bytes in data, so it need to be encoded first.
Fixes:
    >>> form = lxml.html.fromstring('<form method="POST">', base_url='http://localhost')
    >>> lxml.html.submit_form(form)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File ".../lxml/html/__init__.py", line 1119, in submit_form
        return open_http(form.method, url, values)
      File ".../lxml/html/__init__.py", line 1140, in open_http_urllib
        return urlopen(url, data)
      File ".../urllib/request.py", line 163, in urlopen
        return opener.open(url, data, timeout)
      File ".../urllib/request.py", line 464, in open
        req = meth(req)
      File ".../urllib/request.py", line 1183, in do_request_
        raise TypeError(msg)
    TypeError: POST data should be bytes or an iterable of bytes. It cannot be of type str.