Pin virtualenv<21 in tests because of breaking change incompatible with hatch

Author: JohanMabille

.github/workflows/tests.yml

@@ -38,6 +38,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
+      - name: Pin virtualenv to pre-breaking major version
+        run: pipx inject hatch "virtualenv<21"
       - name: Run Tests
         run: hatch run cov:test
       - uses: jupyterlab/maintainer-tools/.github/actions/upload-coverage@v1