fix: grab correct version of flit-core when building tar file

[sgaist]

Summary

The script currently grabs the latest version of flit-core
which at this time does not match the constraints set in the
pyproject.

Use the build-system requires entry to download a version that
matches the constraints.

Use Cases

Fix the compiled dependencies automated update job.
The job currently succeeds but creates an unusable tar ball.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[jericop]

Approving as a fix, but should we have the script support installing all dependencies in the ["build-system"]["requires"] list rather than only [0]?

[sgaist]

I though about it but after reading https://packaging.python.org/en/latest/guides/writing-pyproject-toml/ where most of the time you have only the build backend listed, I went for the simplest solution.

I don't mind adding support to manage a list of more than one dependency though.

[sgaist]

@jericop I have implemented a version that will take all constraints in paketo-buildpacks/python-package-managers-install#289.
Tell me what you think about it and I will backport it here.

[jericop]

I have implemented a version that will take all constraints in paketo-buildpacks/python-package-managers-install#289.
Tell me what you think about it and I will backport it here.

That looks good to me. I'm happy with either approach after reading through the docs, but the latest approach should future-proof us if things change.