Update CHANGELOG

Update changelog

Prepare changelog for release

More updates from putup -U

Make extension "not persistable"

Transitional version (v3.x -> v4.x)

After separating pyscaffoldext-django from the main PyScaffold
repository this version is intended to be used as a dependency to
guarantee backward compatibility.

After the release of PyScaffold v4.0, this dependency will be treated as
optional.