bpo-36611: Fix test_sys.test_getallocatedblocks() (GH-12797)

bpo-36611: Fix test_sys.test_getallocatedblocks() (GH-12797)

Fix test_sys.test_getallocatedblocks() when tracemalloc is enabled.
If the name of Python memory allocators cannot get read, consider
that pymalloc is disabled.

Fix the following error:

./python -X tracemalloc -m test test_sys -v -m test_getallocatedblocks

ERROR: test_getallocatedblocks (test.test_sys.SysModuleTest)
------------------------------------------------------------
Traceback (most recent call last):
  File "Lib/test/test_sys.py", line 770, in test_getallocatedblocks
    alloc_name = _testcapi.pymem_getallocatorsname()
RuntimeError: cannot get allocators name
(cherry picked from commit 9b8314cfe29ca532fc335277f6c36b72e6132922)

Co-authored-by: Victor Stinner <vstinner@redhat.com>