github-actions: add codecov upload

Author: miri64

.github/workflows/tox.yml

@@ -30,3 +30,7 @@ jobs:
         pip install tox tox-gh-actions
     - name: Test with tox
       run: tox
+    - name: Upload to coverage
+      uses: codecov/codecov-action@v1
+      with:
+        flags: ${{ matrix.python-version }}