[2.7] bpo-31692: Add PYTHONSHOWALLOCCOUNT env var (GH-3927)

bpo-31692, bpo-19527:

* Add a new PYTHONSHOWALLOCCOUNT environment variable, similar to
  the Python 3 "-X showalloccount" option
* When Python is compiled with COUNT_ALLOCS, the new
  PYTHONSHOWALLOCCOUNT environment variable now has to be set to dump
  allocation counts into stderr on shutdown. Moreover, allocations
  statistics are now dumped into stderr rather than stdout.
* Add @test.support.requires_type_collecting decorator: skip test if
  COUNT_ALLOCS is defined
* Fix tests for COUNT_ALLOCS: decorate some methods with
  @requires_type_collecting
* test_sys.test_objecttypes(): update object type when COUNT_ALLOCS
  is defined

Python/pythonrun.c

@@ -488,7 +488,9 @@ Py_Finalize(void)
 
     /* Debugging stuff */
 #ifdef COUNT_ALLOCS
-    dump_counts(stdout);
+    if (Py_GETENV("PYTHONSHOWALLOCCOUNT")) {
+        dump_counts(stderr);
+    }
 #endif
 
     _PyDebug_PrintTotalRefs();