Only try ujson over python 3.6

Author: tonybaloney

.github/workflows/gh-tests-ci.yml

@@ -27,6 +27,7 @@ jobs:
       run: |
         pytest --cache-clear --cov=./azure --cov-report=xml --cov-branch tests
     - name: Test with pytest and ujson
+      if: ${{ matrix.python-version }} > 3.6
       run: |
         python -m pip install .[ujson]
         pytest --cache-clear tests