add mepo, update py-pyyaml

[mathomp4]

As spack#44879 seems to be weirdly unmergable (?), just in case, I'll make a PR here with the mepo update from spack#44879 and the py-pyyaml update from spack#44906.

I'll keep monitoring spack#44879 in case that does get in...

[climbfuji]

Since the spack PR was merged, do you want to update this PR so that we have the final version WITH the correct hash from the squashed merged from spack develop?

[mathomp4]

Since the spack PR was merged, do you want to update this PR so that we have the final version WITH the correct hash from the squashed merged from spack develop?

Hmm. Sure...but how would one do that? Always willing to learn new Git routines!

[climbfuji]

Start with spack-stack-dev, configure the spack (authoritative) remote, then cherry-pick the squashed-merge hash from the mepo PR. This will give you a lot of conflicts (because we already have a mepo package for v1). Then git checkout "theirs" (i.e. the spack develop version) so that mepo in spack-stack-dev is identical to that in spack develop. "git add", "git commit". That should do?

[mathomp4]

Superseded by #448

@climbfuji can you take a look at that?