The compilation failure is a consequence of the changes made in issue 30860. Simply adding '#include "internal/pystate.h"' in Modules/_elementtree.c fixes the compilation although this is not the correct fix.

The modules configured in Modules/Setup keep being built with -DPy_BUILD_CORE while the refactoring done in issue 30860 imposes new constraints on the way headers are handled for modules accessing the Py_BUILD_CORE API. Most modules configured in Modules/Setup do not use this API and none of the commented out modules in this file (normally built by setup.py [1]) does. PR 6489 fixes this by introducing yet another CFLAGS named PY_NO_CORE_CFLAGS to only use -DPy_BUILD_CORE with Setup modules that use the Py_BUILD_CORE API.

[1] the _xxsubinterpreters module is the only one that sets -DPy_BUILD_CORE explicitly in setup.py