fix virtualenv constraint (PEP 440 compliant)

[radoering]

#6376 made poetry uninstallable via pip 🙈 (Seems we are missing some kind of smoke test that builds poetry and tries to install it.)

This PR fixes this regression by making the constraint PEP 440 compliant.

[poetry-bot]

The backport to 1.2 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-1.2 1.2
# Navigate to the new working tree
cd .worktrees/backport-1.2
# Create a new branch
git switch --create backport-6402-to-1.2
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 18dbb92d97206ff1e1e9d4625f2b973e80428644
# Push it to GitHub
git push --set-upstream origin backport-6402-to-1.2
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-1.2

Then, create a pull request where the base branch is 1.2 and the compare/head branch is backport-6402-to-1.2.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.