fix conditional codecov

Author: xgui3783

.github/workflows/tox.yaml

@@ -23,7 +23,7 @@ jobs:
         tox
 
     # only 3.6 is run with codecov
-    - if: ${{ matrix.python-version }} == '3.6'
+    - if: ${{ matrix.python-version == '3.6' }}
       uses: codecov/codecov-action@v2
       with:
         fail_ci_if_error: true # optional (default = false)