Orange3-SingleCell can be installed directly from Orange add-on manager. You can find add-on manager in Options menu.
If you are using python provided by Anaconda distribution, you are almost ready to go. Add conda-forge to the list of channels you can install packages from
conda config --add channels conda-forge
conda install orange3-singlecell
Orange3-SingleCell can also be installed from the Python Package Index:
pip install orange3-singlecell