Pin version of Poetry in CI to 1.5.1 to maintain support for Python 3.7

[Nour-Mws]

Closes #352

[jherland]

Looks good. Thanks for fixing! 🚀

It occurs to me that outside of GitHub CI, we also don't really constrain the Poetry version. Sure, if you use our Nix shell, we have pinned the nixpkgs version in shell.nix, which in effect also pins Poetry at whatever version is used in that nixpkgs. But if you don't use Nix, then all bets are off.

I looked for a way to constrain the Poetry version from within pyproject.toml (it would not actually cause the right Poetry version to be used, but would at least fail loudly if the wrong version is used). However, some searching found python-poetry/poetry#6644 (and python-poetry/poetry#3316) which suggest that this is not a high priority for the Poetry team, especially not the part about restricting a newer Poetry from being used.

So in the end, I think there is not much else we can/should do for the non-GitHub situation.