Add tomli as a dependency in GitHub Actions CI

Adding tomli to fix the GitHub Actions CI test
breakages reported in #1392. This is a temporary
measure until `pytest-cov>2.12.1` is released.

Author: weiji14

.github/workflows/ci_tests.yaml

@@ -97,7 +97,7 @@ jobs:
                         pandas xarray netCDF4 packaging \
                         ${{ matrix.optional-packages }} \
                         coverage[toml] dvc=2.3.0 make pytest>=6.0 \
-                        pytest-cov pytest-mpl sphinx-gallery
+                        pytest-cov pytest-mpl sphinx-gallery tomli
 
       # Show installed pkg information for postmortem diagnostic
       - name: List installed packages