pysss: Fix DeprecationWarning PY_SSIZE_T_CLEAN
    
    src/tests/pysss-test.py:73: DeprecationWarning: PY_SSIZE_T_CLEAN will be required for '#' formats
      val1 = obfuscator.encrypt("123", obfuscator.AES_256)
    
    These were introduced by https://bugs.python.org/issue36381 to warn about
    an upcoming Python C API change. The meaning of PY_SSIZE_T_CLEAN is described
    in https://python.readthedocs.io/en/stable/c-api/arg.html#strings-and-buffers.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>\