Revert codecov/codecov-action back to v3 (isce-framework#33)

Revert isce-framework#32

This update caused CI to break after merging, with error

```
Unable to resolve action `codecov/codecov-action@v4`, unable to find
version `v4`
```

The codecov-action v4 releases appear to be unstable pre-releases
currently. Maybe one of these was temporarily marked as a stable release
by accident, triggering the dependabot update?

.github/workflows/pytest.yml

@@ -52,4 +52,4 @@ jobs:
           micromamba install -f test/requirements.txt -c conda-forge
           pytest --cov-report=xml
       - name: Coverage report
-        uses: codecov/codecov-action@v4
+        uses: codecov/codecov-action@v3