Support older py-pygments (#25456)
Support older py-pygments (#25456)

`markdown` is only supported since py-pygments@2.8.0:, see
https://github.com/pygments/pygments/commit/9647d2ae506b8e05ebabe9243df707bac901a6a3

Let's allow old versions too again.
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack