This is issue introduced with implementation of SOABI. Build of standard extensions is protected by following code:
-----
class PyBuildExt(build_ext):

    def __init__(self, dist):
        build_ext.__init__(self, dist)
        self.failed = []

    def build_extensions(self):

        # Detect which modules should be compiled
        old_so = self.compiler.shared_lib_extension
        # Workaround PEP 3149 stuff
        self.compiler.shared_lib_extension = os.environ.get("SO", ".so")
        try:
            missing = self.detect_modules()
        finally:
            self.compiler.shared_lib_extension = old_so
....
------

I think that PEP 3149 is not  accurate . For historical reasons (backward compatibility) SO must remain same as OS specific suffix and and new variable is required for python specific suffix.