The new subprocess module test is not compatible with a Python built --with-pydebug because the [xxxx refs] output breaks the comparison.
I've recently been thinking that it would be nice if that [xxxx refs] output could be suppressed with an envar or a command line switch. FWIW, I generally build our embedding product with a --with-pydebug built Python and that output breaks our unit tests too. I work around this by suppressing the stderr comparison in our testing framework.
Once we decide to do it, and what to wrap that call in, it's likely a one or two line change to pythonrun.c. Does one of the existing envars make sense to use?
Barring any change here, we need to at least make sure the subprocess test doesn't fail for a debug build.