Pin tox-gh and tox versions for CI

They are both still unstable.

Author: amorison

.github/workflows/tox.yml

@@ -28,6 +28,6 @@ jobs:
       - name: Update pip
         run: python -m pip install -U pip
       - name: Install tox-gh
-        run: python -m pip install tox-gh
+        run: python3 -m pip install tox-gh==0.0.3 tox==4.0.0a9
       - name: Run tox
         run: tox4 run