Thanks for checking, it's a pity that os.putenv() does not work.

There's a global variable malloc_logger that is set from the
environment on startup:

https://opensource.apple.com/source/libmalloc/libmalloc-53.1.1/src/malloc.c


It seems to be always checked for NULL, but I can't find any
documentation that says it is guaranteed.

A nuclear option would be to set malloc_logger to NULL on Python
startup.  Or write a custom malloc_logger, which *does* seem to
be supported.