Add XpressPyomoSolver

.github/workflows/test.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.6, 3.7, 3.8]
 
     steps:
     - name: Check out source code
@@ -19,6 +19,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
+        python -m pip install xpress
         python -m pip install -i https://pypi.gurobi.com gurobipy
         pip install -r requirements.txt