Fix tests, add six dependency

Author: Pike

.github/workflows/syntax-tests.yml

@@ -24,7 +24,7 @@ jobs:
         run: |
           python -m pip install wheel
           python -m pip install --upgrade pip
-          python -m pip install .
+          python -m pip install six
       - name: Test
         working-directory: ./fluent.syntax
         run: |