SpecList: remove mutable types from __init__ arguments (#18515)
SpecList: remove mutable types from __init__ arguments (#18515)

fixes #18439
iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack