Build broken on Python 3.10 #169

/usr/local/include/python3.10/ceval.h:17:43: note: declared here
     Py_DEPRECATED(3.9) PyAPI_FUNC(PyObject *) PyEval_CallObjectWithKeywords(
                                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    greenlet.c: In function ‘green_dealloc’:
    greenlet.c:1009:19: error: lvalue required as left operand of assignment
       Py_REFCNT(self) = 1;
                       ^
    greenlet.c:1038:7: error: lvalue required as decrement operand
       if (--Py_REFCNT(self) != 0) {
           ^~
    greenlet.c:1042:20: error: lvalue required as left operand of assignment
        Py_REFCNT(self) = refcnt;