Add codecov.io publishing (#204)

Author: nfx

.github/workflows/push.yml

@@ -39,3 +39,8 @@ jobs:
       - name: Run unit tests
         run: |
           hatch run unit:test
+
+      - name: Publish test coverage
+        uses: codecov/codecov-action@v3
+        with:
+          files: .coverage