Good news, everyone! The gmxapi 0.2 Python package is available for GROMACS 2021, and works best with the brand new release of GROMACS 2021.1.
After activating your new GROMACS installation ("source"ing your GMXRC), you should be able to just do
pip install --upgrade gmxapi in your virtual environment. See also Full installation instructions — GROMACS 2021.1 documentation.
If you are using gmxapi to connect pluggable MD extension code to GROMACS simulations, continued compatibility should be easy, but may require some updates. The sample code at GitHub - kassonlab/sample_restraint has been confirmed to work with all gmxapi and supported GROMACS versions, and should be instructive.