urllib.request.urlopen from future.backports doesn't accept data URLs

[sagittarian]

Passing a data URL (such as "data:text/plain,hello") to urlopen raises a URLError, although this works in both Python 2.7 and Python 3.5. I'm not sure if this was added after Python 3.3 on which the backports code is based according to the source, but according to the 3.3 documentation it seems that it should have worked even in Python 3.3.

[sagittarian]

Also, URLopener().open(data_url)/.open_data(data_url) add some headers to the data returned by .read(), which doesn't happen in the stdlib versions, and doesn't seem like it would be very useful to someone trying to get data out of a data url.