Use `univers` for sorting out the latest version of a package

[keshav-space]

get_latest_version should use univers to sort out the latest version, instead of using natsort. Natsort should be a fallback method for ecosystem not yet supported in univers.
https://github.com/nexB/purldb/blob/e1a20006b513a0379e69a69624da4e614a4f27dd/packagedb/models.py#L612-L618