Do we want to consider these pre-release tasks or post-release tasks? We don't have officially codified post-release tasks right now, but maybe we need some?

* Update to next version (e.g. 0.5.dev1)
* Update CHANGELOG to add master header again
* Update items in deprecation cycle (see `utils.py`). This either requires changing `PendingDeprecationWarning` to `DeprecationWarning` or the more extensive "remove everything" task. Maybe the post-release task should just be to file an issue to process through deprecation cycle items?