Fix tests on macOS

To be honest, I don’t know how it used to work. Using setup-python’s version of Python
doesn’t set the environment variable set by Homebrew’s Python to use Homebrew’s
libraries.

Don’t try this at home, because you don’t need setup-python. Just use Homebrew’s Python.

.github/workflows/tests.yml

@@ -41,6 +41,8 @@ jobs:
         run: python -m pip install .[test] pytest-xdist
       - name: Launch tests
         run: python -m pytest -n auto
+        env:
+          DYLD_FALLBACK_LIBRARY_PATH: /opt/homebrew/lib
       - name: Check coding style
         run: python -m flake8
       - name: Check imports order