Fix for --skip-lock

[matteius]

Thank you for contributing to Pipenv!

The issue

Fixes #5367

Depends on: https://github.com/sarugaku/plette/pull/26/files

The checklist

• Associated issue
• A news fragment in the news/ directory to describe this fix with the extension .bugfix.rst, .feature.rst, .behavior.rst, .doc.rst. .vendor.rst. or .trivial.rst (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.

[oz123]

Plette has now version 0.3.1, and we can also add a test for this case.

I can confirm that this fix works.