Bump cel-python version

[o-murphy]

Bump cel-python version, the latest one is 0.4.0

[stefanvanburen]

Hey @o-murphy, agreed, I'd like to do that, but have previously tried to upgrade to 0.4.0 and 0.3.0 and unfortunately there was a regression introduced in 0.3.0 that hasn't been fixed. Ideally we'd fix that upstream before pulling in the new version.

Going to leave this ticket open, though, so people can see this.