text: Title: Not compatible with Cython 3.1 due to Py2 code
  Body: Cython 3.1 removes Py2 support, but gevent still has Py2 code in its code base, e.g.
  
  https://github.com/gevent/gevent/blob/07d2b7e4a3c5881e0a3637062b3c7c7c0027971c/src/gevent/libev/corecext.pyx#L64-L69
  
  This leads to build failures for users who have Cython 3.1 installed, as described in
  
  https://github.com/cython/cython/issues/6490
  
  A quick fix would be to exclude Cython 3.1 from the build dependencies:
  
  https://github.com/gevent/gevent/blob/07d2b7e4a3c5881e0a3637062b3c7c7c0027971c/pyproject.toml#L21
  
  However, since Python 2.x support seems no longer intended for gevent, removing the Py2 left-overs seems the best path forward.