Fix the internals of our hash functions to used unsigned values during hash

computation as the overflow behavior of signed integers is undefined.

In practice we require compiling everything with -fwrapv which forces overflow
to be defined as twos compliment but this keeps the code cleaner for checkers
or in the case where someone has compiled it without -fwrapv or their
compiler's equivalent.

Found by Clang trunk's Undefined Behavior Sanitizer (UBSan).

Cleanup only - no functionality or hash values change.

Objects/bytesobject.c

@@ -873,7 +873,7 @@ bytes_hash(PyBytesObject *a)
 {
     register Py_ssize_t len;
     register unsigned char *p;
-    register Py_hash_t x;
+    register Py_uhash_t x;  /* Unsigned for defined overflow behavior. */
 
 #ifdef Py_DEBUG
     assert(_Py_HashSecret_Initialized);