Add basic support for uproot4
This PR adds basic support for uproot >=4. It's hacky because uproot 4 doesn't support writing ROOT files yet, so there's an import hook in place to advise users to install uproot3 to do that if they need to.
This PR adds basic support for uproot >=4. It's hacky because uproot 4 doesn't support writing ROOT files yet, so there's an import hook in place to advise users to install uproot3 to do that if they need to.