PRs #303 and #304 show that there's a 0.3.1 version of CellBender, but the corresponding commit was not tagged (cfr. https://github.com/broadinstitute/CellBender/tags), and hence there's also no corresponding 0.3.1 release in GitHub (cfr. https://github.com/broadinstitute/CellBender/releases).
In addition, there's no 0.3.1 release available via PyPI either, see https://pypi.org/project/cellbender/#history
Is this deliberate for some reason?