[pgp] Fixed file sharing feature.

Hello, this is a small patch for the PGP legacy plugin. There was a small problem when sending a file: The encrypt_file method from PGP module needed a stream (file instance) instead of bytes, it was probably due to some gajim updates when file.get_data was returning a file/stream. I guess this would need also to be updated on master but I couldn't do a pull request for multiple branches.

Also, I've just noticed now, but it doesn't seem to support decrypting files on the fly. Though, I'm not sure if I should contribute more to this plugin since XEP-0027 is now archived. Should I go instead for the new OpenPGP XEP-0373 plugin to support file sharing ?

I'm asking this because some others clients does not have yet the experimental XEP-0373, so I'm kinda hesitating. But maybe this would promote the newer XEP, so others clients implements it (Conversations for example).