chore(internal): resolve incorrect import resolution from Cython

[brettlangdon]

Description

This change is a follow-up to #4085 to provide a more long term solution to the problem and reverting the pinning of setuptools versions.

setuptools==64.0.0 changed the way editable modules are linked to in site-packages (from .egg-link to .pth + a loader). This exposed and issue with the import of ddtrace.constants from within a Cython module.

Cython was loading the ddtrace.internal.constants module instead.

See #4085 and https://github.com/brettlangdon/shadow-import-issue for more details.

Checklist

• [x] Title must conform to conventional commit.
• [x] Add additional sections for feat and fix pull requests.
• [x] Ensure tests are passing for affected code.
• [x] Library documentation and/or Datadog's documentation site is updated. Link to doc PR in description.

Reviewer Checklist

• [ ] Title is accurate.
• [ ] Description motivates each change.
• [ ] No unnecessary changes were introduced in this PR.
• [ ] PR cannot be broken up into smaller PRs.
• [ ] Avoid breaking API changes unless absolutely necessary.
• [ ] Tests provided or description of manual testing performed is included in the code or PR.
• [ ] Release note has been added for fixes and features, or else changelog/no-changelog label added.
• [ ] All relevant GitHub issues are correctly linked.
• [ ] Backports are identified and tagged with Mergifyio.
• [ ] Add to milestone.