Add version-independent python directory

[erozen]

🗣 Description

Add a symlink /opt/venv/lib/python that points to the versioned python directory next to it.

💭 Motivation and context

With this change, the config file can reference skins and other directories under the venv without requiring path updates between python upgrades.

🧪 Testing

Manually created symlink in running container using same command.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced
  in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Add a tag or create a release.