bpo-31217: Fix regrtest -R for small integer (#3260)

Use a pool of integer objects toprevent false alarm when checking for
memory block leaks. Fill the pool with values in -1000..1000 which
are the most common (reference, memory block, file descriptor)
differences.
Co-Authored-By: Antoine Pitrou <pitrou@free.fr>

Lib/test/libregrtest/refleak.py

@@ -38,6 +38,14 @@ def dash_R(the_module, test, indirect_test, huntrleaks):
         for obj in abc.__subclasses__() + [abc]:
             abcs[obj] = obj._abc_registry.copy()
 
+    # bpo-31217: Integer pool to get a single integer object for the same
+    # value. The pool is used to prevent false alarm when checking for memory
+    # block leaks. Fill the pool with values in -1000..1000 which are the most
+    # common (reference, memory block, file descriptor) differences.
+    int_pool = {value: value for value in range(-1000, 1000)}
+    def get_pooled_int(value):
+        return int_pool.setdefault(value, value)
+
     nwarmup, ntracked, fname = huntrleaks
     fname = os.path.join(support.SAVEDCWD, fname)
     repcount = nwarmup + ntracked
@@ -56,9 +64,9 @@ def dash_R(the_module, test, indirect_test, huntrleaks):
                                                          abcs)
         print('.', end='', file=sys.stderr, flush=True)
         if i >= nwarmup:
-            rc_deltas[i] = rc_after - rc_before
-            alloc_deltas[i] = alloc_after - alloc_before
-            fd_deltas[i] = fd_after - fd_before
+            rc_deltas[i] = get_pooled_int(rc_after - rc_before)
+            alloc_deltas[i] = get_pooled_int(alloc_after - alloc_before)
+            fd_deltas[i] = get_pooled_int(fd_after - fd_before)
         alloc_before = alloc_after
         rc_before = rc_after
         fd_before = fd_after