Installing With Conda
If you use conda (normally via the Anaconda Python distribution), then all of the prerequisites and galsim itself are available from the conda-forge channel, so you can use that as follows:
conda install -c conda-forge galsim
Also, if you prefer to use the defaults channel, then (at least as of this writing), it had all the items in conda_requirements.txt, except for pybind11. So if you have conda-forge in your list of channels, but it comes after defaults, then that should still work and pybind11 will be the only one that will need the conda-forge channel.
If you want to install from source (e.g. to work on a development branch), but use conda for the dependencies, you can do:
git clone git@github.com:GalSim-developers/GalSim.git
cd GalSim
conda install --file conda_requirements.txt
python setup.py install