Add missing Py_REFCNT import for Cython compilation

Author: cxzhong

src/cysignals/signals.pyx

@@ -25,7 +25,7 @@ See ``tests.pyx`` for extensive tests.
 
 from libc.signal cimport *
 from libc.stdio cimport freopen, stdin
-from cpython.ref cimport Py_XINCREF, Py_CLEAR
+from cpython.ref cimport Py_XINCREF, Py_CLEAR, Py_REFCNT
 from cpython.exc cimport (PyErr_Occurred, PyErr_NormalizeException,
         PyErr_Fetch, PyErr_Restore)
 from cpython.version cimport PY_MAJOR_VERSION