Whether or not a macOS sysroot path is included in CFLAGS depends on how the Python build was configured and built. For instance, the macOS Python binaries provided on python.org do not include a sysroot path in CFLAGS; from the version info provided, you are using a Python from some other source. But, even if there is one there, there is code in the standard library helper function _osx_support.py to remove a sysroot from CFLAGS at runtime if the file is not present. That is used by distutils and *should* be being used by setuptools as well but it's possible that other build systems aren't using it or there might be a bug there. Can you give a reproducible test case showing a build failure including from where you got the Python 3.9.7 you are using?

https://github.com/python/cpython/blob/3.9/Lib/_osx_support.py#L331