Bump codecov action version

This commit bumps the codecov action version. We were previously using
v3 which no longer is supported and can't be used anymore. This upgrades
the action to use the latest version v5.

.github/workflows/main.yml

@@ -83,7 +83,7 @@ jobs:
       - name: Run coverxml
         run: tox -ecoverxml
       - name: codecov
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v5
         with:
           files: ./cover/coverage.xml # optional
           flags: unittests # optional