This is not actually a bug. MINGW64 build is not officially supported, but as I need it I got my PR merged. This PR still won't fully fix the problem. We also need this commit, but the maintainers are

Expected behavior pip3 install neovim should work on mingw64 mintty session. Actual behavior Neovim python client requires greenlet. Because greenlet does not support mingw64, it crashes as greenlet mistakenly