notably, C99 variable length arrays syntax is not mentioned as allowed in https://www.python.org/dev/peps/pep-0007/.  If we want to use VLAs, that should be clarified.  But both our solutions should work with [1] instead of [] which is allowed by the ubsan checker.

it means keeping the annoying "- Py_MEMBER_SIZE(...)" of the array on all size calculations (but because of that the patch is smaller).