Roll back to codecov/codecov-action@v3 action

v4 stopped supporting tokenless uploads.

Author: lpsinger

.github/workflows/pull_request.yml

@@ -43,4 +43,4 @@ jobs:
         run: poetry run mypy .
 
       - name: Upload to Codecov.io
-        uses: codecov/codecov-action@v4
+        uses: codecov/codecov-action@v3