Add full path for PyCell

[mejrs]

This makes it so that users no longer have to bring PyCell into scope to make the #[pyclass] macro work (either with the prelude or explicit import).

[messense]

You need to update UI tests output with TRYBUILD=overwrite cargo test

[davidhewitt]

Thanks! I'll do this myself and force-push now, because this looks like a very good catch that would be a potentially annoying bug in 0.14.