Yes, python-sphinx is the Python 3 version of Sphinx. Note that Guix does not conform to the FHS, and thus has no /usr directory.
In Fedora we have (most recent change in packaging guidelines):
Source rpm: python-foo which generates binary rpms: python2-foo and python3-foo where python2-foo has virtual provides for python-foo, once Python3 will become THE default python by the upstream merits we will change python-foo provides to python3-foo package.