update github unittest workflow

Author: smoors

.github/workflows/unittest.yml

@@ -12,7 +12,7 @@ jobs:
       with:
         python-version: ${{ matrix.python }}
     - name: install tox
-      run: pip install tox
+      run: pip install 'virtualenv<20.22.0' 'tox<4.5.0'
     - name: add mandatory git remote
       run: git remote add hpcugent https://github.com/hpcugent/vsc-base.git
     - name: Run tox