@Farman_Sayem thanks for reaching out. There’s a separate ongoing thread regarding the specific 403 error you face. We’re trying to narrow it down over there, and would appreciate any helpful feedback you could add.
As for the new API client supporting python 3.7, that’s highly unlikely (@munrojm). End of life for python 3.7 is tomorrow which means that the developers of diffpycmi would really need to consider moving to a newer python version asap.