build: 💚 Fix building of matrix build via poetry

See also: microsoft/debugpy#1246 (comment)

Author: Anselmoo

.github/workflows/python-ci.yml

@@ -55,7 +55,9 @@ jobs:
           virtualenvs-path: .venv
           installer-parallel: true
       - name: Install library
-        run: poetry install --no-interaction --with dev --all-extras
+        run: |
+          poetry config installer.modern-installation false
+          poetry install --no-interaction --with dev --all-extras
       - name: Test SpectraFit
         run: poetry run pytest
       - name: Codecov