[CI] Restore running Python backend tests which rely on mock QPU (#3541)

Signed-off-by: Pradnya Khalate <[email protected]>

Author: khalatepradnya

.github/workflows/test_in_devenv.yml

@@ -238,10 +238,10 @@ jobs:
             apt-get update -y && apt-get install -y python3-venv
 
             VENV_DIR="$HOME/.venvs/cudaq"
-            python3 -m venv "$VENV_DIR"
+            python3 -m venv --system-site-packages "$VENV_DIR"
             . "$VENV_DIR/bin/activate"
 
-            pip install iqm-client -vvv
+            pip install iqm-client==16.1
             pip install . -vvv
             pyinstall_status=$?
             if [ ! $pyinstall_status -eq 0 ]; then