bpo-32030: Rework memory allocators (#4625)

* Fix _PyMem_SetupAllocators("debug"): always restore allocators to
  the defaults, rather than only caling _PyMem_SetupDebugHooks().
* Add _PyMem_SetDefaultAllocator() helper to set the "default"
  allocator.
* Add _PyMem_GetAllocatorsName(): get the name of the allocators
* main() now uses debug hooks on memory allocators if Py_DEBUG is
  defined, rather than calling directly malloc()
* Document default memory allocators in C API documentation
* _Py_InitializeCore() now fails with a fatal user error if
  PYTHONMALLOC value is an unknown memory allocator, instead of
  failing with a fatal internal error.
* Add new tests on the PYTHONMALLOC environment variable
* Add support.with_pymalloc()
* Add the _testcapi.WITH_PYMALLOC constant and expose it as
   support.with_pymalloc().
* sysconfig.get_config_var('WITH_PYMALLOC') doesn't work on Windows, so
   replace it with support.with_pymalloc().
* pythoninfo: add _testcapi collector for pymem

Lib/test/test_sys.py

@@ -753,8 +753,15 @@ class SysModuleTest(unittest.TestCase):
     @unittest.skipUnless(hasattr(sys, "getallocatedblocks"),
                          "sys.getallocatedblocks unavailable on this build")
     def test_getallocatedblocks(self):
+        try:
+            import _testcapi
+        except ImportError:
+            with_pymalloc = support.with_pymalloc()
+        else:
+            alloc_name = _testcapi.pymem_getallocatorsname()
+            with_pymalloc = (alloc_name in ('pymalloc', 'pymalloc_debug'))
+
         # Some sanity checks
-        with_pymalloc = sysconfig.get_config_var('WITH_PYMALLOC')
         a = sys.getallocatedblocks()
         self.assertIs(type(a), int)
         if with_pymalloc: