cython 3.1.0 removes cpython.string

see https://github.com/cython/cython/pull/5870

With cython >=3.1.0 (released since May 8th, 2025), calcephpy produces the error at the installation

      Error compiling Cython file:
      ------------------------------------------------------------
      ...
      # see http://docs.cython.org/en/latest/src/tutorial/clibraries.html
      # compile with : python setyup.py build_ext -i
      cimport calcephpy
      from cpython.mem cimport PyMem_Malloc, PyMem_Realloc, PyMem_Free
      cimport cpython.version
      cimport cpython.string
              ^
      ------------------------------------------------------------
      
      pythonapi/src/calcephpy_after_3_0.pyx:66:8: 'cpython/string.pxd' not found