Fix release tests

actions/setup-python#866 (comment)

Author: jacobtylerwalls

.github/workflows/release-tests.yml

@@ -20,6 +20,8 @@ jobs:
         with:
           # virtualenv 15.1.0 cannot be installed on Python 3.10+
           python-version: 3.9
+        env:
+          PIP_TRUSTED_HOST: "pypi.python.org pypi.org files.pythonhosted.org"
       - name: Create Python virtual environment with virtualenv==15.1.0
         run: |
           python -m pip install virtualenv==15.1.0