Use %py3_test_envvars in %tox when available
    
    The new %py3_test_envvars macro was added
    to remove duplication of environment variables used in %check.
    
    We reuse it in %tox to gain support e.g. for PYTEST_XDIST_AUTO_NUM_WORKERS.
    
    However, we keep support for platforms where the macro is not yet available,
    not to be forced to backport %py3_test_envvars everywhere right away.
    
    Technically, this should change little, but it sets CFLAGS and LDFLAGS now,
    hence a new Y version.