From the PR comment thread (as I opened that first):

"""Well, there was no other choice in ISO C89 than using char ob_sval[1];, no?

Is char ob_sval[]; supported by the C compiler supported by CPython? Like Visual Studio, GCC, clang and xlc (AIX)? (I don't think that we officially support xlc, but it's more "best effort" support.)

You can use the new buildbot label to test you change on more platforms.""" - vstinner

Per https://www.python.org/dev/peps/pep-0007/ we require some C99 features as of CPython 3.6.  It does not currently list Flexible array member.

I'll be very surprised if we find any compiler that does not support this.

I'll run this through the buildbot testing as you suggested and assuming nothing important falls out, see that we add this to the C99 required feature list.