Use pymodbus 3.6.9 in itests

.github/workflows/build.yml

@@ -32,7 +32,7 @@ jobs:
     - name: Install test dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install setuptools wheel twisted pyserial pymodbus
+        pip install setuptools wheel twisted pyserial pymodbus==3.6.9
         sudo apt-get install -y socat
 
     - name: Run tests