Fix PyObject_Hash signature in comment (#4905)

diff --git a/Include/abstract.h b/Include/abstract.h
index 991c1c3..3133cd1 100644
--- a/Include/abstract.h
+++ b/Include/abstract.h
@@ -367,7 +367,7 @@
 
 /* Implemented elsewhere:
 
-   long PyObject_Hash(PyObject *o);
+   Py_hash_t PyObject_Hash(PyObject *o);
 
    Compute and return the hash, hash_value, of an object, o.  On
    failure, return -1.