The recent releases of PyMOL (2.3.*) use Python 3.7.
Support for Python2 ends Dec. 31 2019.
It would be great to make available a Python3 version of sassy.py.
Request for Python3 version of saspy.py
Try ATSAS 3.0
SASpy from ATSAS 3.0 works with Python 2.7 as well as with Python 3.7, please upgrade.