Lazy load zstandard

[mattwthompson]

Description

This PR defers import zstandard until it's needed. Some weird packaging issues cause a conflict that prevents any QCPortal import. I'm not sure when these compression/caching modules are used, i.e. by common user operations or only by admins. If they're ever not needed, this avoids an unnecessary crash.

Changelog description

Lazy load zstandard

Status

• Code base linted
• Ready to go

[bennybp]

Playing around with the zstd stuff, I see they have an environment variable for how the backend gets loaded. It seems to help in my case. https://python-zstandard.readthedocs.io/en/latest/api_usage.html

$ PYTHON_ZSTANDARD_IMPORT_POLICY='cffi_fallback' python3 -c 'from openeye import oechem; import zstandard;print("ok")'
ok

So openeye bundles a zstd dynamic library with it? I see a ./libs/python3-linux-x64-g++10.x/libzstd.so.1 in the openeye package. I think using cffi would then make zstandard imports use the installed library that's part of the zstd package, but I'm not positive and no idea if that would cause problems.

[mattwthompson]

I wish I found that a while back - that's exactly what I need!