That error is generally caused by a setup script using distutils attempting to set the $MACOSX_DEPLOYMENT_TARGET environment variable to a value lower than the value which was set during the compilation of the Python running the script.
The setup.py file in the Aerospike Python client repo currently sets that variable to ‘10.11’
and I believe that is what is causing the error when attempting to build it from there.
The first fix I would try is to change the line in setup.py:
os.environ['MACOSX_DEPLOYMENT_TARGET'] = '10.11'
os.environ['MACOSX_DEPLOYMENT_TARGET'] = '10.12'
and try to rerun the build command e.g. python setup.py install