installs pyomo in github workflow

.github/workflows/CI.yml

@@ -36,7 +36,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install flake8 pytest
+        pip install flake8 pytest pyomo==4.6.1
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
     - name: Install CBC
       run: |