ci: install xenon explicitly for complexity gate

Author: TartzBrewer

.github/workflows/ci.yml

@@ -27,6 +27,7 @@ jobs:
           python -m pip install --upgrade pip
           pip install -e .[dev-tools]
           pip install mypy
+          pip install xenon
       - name: Pre-commit (all files)
         uses: pre-commit/action@v3.0.1
         with: