Go back to caching the watcher pointer.

src/gevent/python.pxd

-cdef extern from "Python.h":
-    struct PyObject:
-        pass
-    ctypedef PyObject* PyObjectPtr "PyObject*"
-    void   Py_INCREF(PyObjectPtr)
-    void   Py_DECREF(PyObjectPtr)
-    void   Py_XDECREF(PyObjectPtr)
-    int    Py_ReprEnter(PyObjectPtr)
-    void   Py_ReprLeave(PyObjectPtr)
-    int    PyCallable_Check(PyObjectPtr)
-
-cdef extern from "frameobject.h":
-    ctypedef struct PyThreadState:
-        PyObjectPtr exc_type
-        PyObjectPtr exc_value
-        PyObjectPtr exc_traceback
-    PyThreadState* PyThreadState_GET()