To my knowledge, Kiri8128 is the first to reach grandmaster with python3/pypy3 without ever switching to C++.

Overall he's the third person to reach GM with python, the other two are pajenegod who uses pypy2 and conqueror_of_tourist who uses pypy3 (but occasionally switches to C++ for tight TL).

Congrats and thanks for being an inspiration for the rest of us python users!