python and highly-dependent packages (Pymatgen etc.) compatiablity

[yuxin-chang]

conflicts
README.md specifies python=3.7; diffpy and diffpy-cmi required python 3.7 and below.
while in setup.py, most package requirements (pymatgen, scikitimage, numpy) support python 3.9 or above

[yuxin-chang]

solution:
first decide if we want to use python 3.7 or 3.9
(considering diffpy dependency)
then update setup.py requirement

[yuxin-chang]

try install with python 3.9 (dont install diffpy)
see if setup.py work