opensubtitles: Fix indentation

Spotted by Ignacio Casal Quinteiro <ignacio.casal@nice-software.com>
opensubtitles: Make decoding download items work

Stop using StringIO, gzip and base64 modules, and use GLib for
the base64 decoding, and zlib for the decompression.

https://bugzilla.gnome.org/show_bug.cgi?id=723074