Fix Tox installing Terra from PyPI

tox.ini

@@ -75,6 +75,11 @@ deps =
   setuptools_rust  # This is work around for the bug of tox 3 (see #8606 for more details.)
   -r{toxinidir}/requirements-dev.txt
   qiskit-aer
+  # Aer depends on Terra. We want to make sure pip satisfies that requirement from a local
+  # installation, not from PyPI. But Tox normally doesn't install the local installation until
+  # after `deps` is installed. So, instead, we tell pip to do the local installation at the same
+  # time as Aer. See https://github.com/Qiskit/qiskit-terra/pull/9477.
+  -e .
 commands =
   sphinx-build -W -j auto -T --keep-going -b html docs/ docs/_build/html {posargs}