Only pull in from requirements.txt

Addresses the same problem as #12439.

Author: David Robertson

debian/build_virtualenv

@@ -55,6 +55,7 @@ dh_virtualenv \
     --preinstall="lxml" \
     --preinstall="mock" \
     --preinstall="wheel" \
+    --extra-pip-arg "--no-deps" \
     --extra-pip-arg="--no-cache-dir" \
     --extra-pip-arg="--compile" \
     --extras="all,systemd,test" \