[WIP] Update our release files

Now when #2318 is in, we need to update our release scripts to work with it.
The change should be along the lines of this PR, but I still need to test this.

TODO:

* [ ] Test that the release works and does what we need
* [ ] Handle the `fabvenv` package in a better way (I don't have it installed and fabfile still seems to work), currently I commented it out, but it should probably only be used in the functions that need it. Need to investigate this.
* [ ] Remove all occurrences of `py3`.