Add missing virtualenv install

Author: MilesCranmer

.github/workflows/tests.yml

@@ -66,7 +66,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install flake8 pytest pytest-cov
+          pip install flake8 pytest pytest-cov virtualenv
           cp pysrc/juliacall/juliapkg-dev.json pysrc/juliacall/juliapkg.json
           pip install -e .
       - name: Lint with flake8