Remove the occurrence of module distutils for Python 3.10

[XuehaiPan]

Module distutils is deprecated in Python 3.10 and will be removed in Python 3.12. distutils will no longer be a Python built-in module (standard library) and have been migrated to setuptools.

See PEP 632 -- Deprecate distutils module.

In this PR:

1. distutils.spawn.find_executable → shutil.which (python >= 3.2)
2. distutils.version.LooseVersion → pkg_resources.parse_version (NOTE: pkg_resources is part of setuptools)
3. distutils.core → setuptools

[lydia-duncan]

This all seems reasonable to me, I'll wait to run testing until you've resolved the merge conflicts (I think it's just a matter of accepting some additions from main to files you've modified)

[ronawho]

Whoops, I missed that @lydia-duncan had already started a review on this (had a stale tab open from this morning.) I left some inline comments. I think the only one that is important to address is reverting the changes to third-party/llvm, the others are more style nits and I would be happy to follow up on those in a separate PR.

[ronawho]

Full std testing looks good:

[Summary: #Successes = 13581 | #Failures = 0 | #Futures = 0]

I think this is ready to merge, thanks @XuehaiPan!

@lydia-duncan any concerns?

[lydia-duncan]

No objections from me, thanks Elliot!