Fix POST form submission for Python 3

[jwilk]

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.

[scoder]

Thanks. Any reason why you chose ASCII instead of UTF-8 as encoding here?

[jwilk]

urlencode() always returns an ASCII-only string, so ASCII as encoding seemed a natural choice.