Motivation for this change

Poetry build with python 3.8 was broken:

$ nix-build -E 'with import ./. {}; poetry.override { python = python38; }'
error: value is null while a set was expected, at /nix/store/24kjp6k6piq42jxvzx80imdx0ysz6grz-nixexprs.tar.xz
  /pkgs/development/tools/poetry2nix/poetry2nix/overrides.nix:569:27 (use '--show-trace' to show detailed location information)

See nix-community/poetry2nix#50 for further info. The proper fix might take a while to be implemented. This here is only a workaround until then.

Ping @adisbladis

This PR is sponsored by Niteo ✨

Things done

• Checked that the normal Python 3.7 poetry still builds: nix-build -A poetry
• Checked that now a Python 3.8 poetry builds with nix-build -E 'with import ./. {}; poetry.override { python = python38; }'