Fix downstream test: do not use editable install for ipyparallel (#1516)

Author: krassowski

.github/workflows/downstream.yml

@@ -69,7 +69,7 @@ jobs:
         uses: jupyterlab/maintainer-tools/.github/actions/downstream-test@v1
         with:
           package_name: ipyparallel
-          package_spec: '-e ".[test]"'
+          package_spec: ".[test]"
 
   jupyter_kernel_test:
     runs-on: ubuntu-latest