Verify pylint behaviour for umpost

[truth-quark]

During the #74 review, it was unclear if the linter workflow step was skipped the umpost.um2netcdf module due to the --ignored-modules option in .github/workflows/CI.yml:

      - name: Lint
        shell: bash -l {0}
        run: pylint --extension-pkg-whitelist=netCDF4 --ignored-modules=um2netcdf4,umpost -E umpost

There is potential that modules in the umpost package are skipped, in addition to um2netcdf4. Given the mid-August release, this was not confirmed due to lack of time.

The main task is ensuring that umpost modules are linted.
Note: this should happen after #78 to remove old um2netcdf4 imports.

[truth-quark]

The original discussion is here: https://github.com/ACCESS-NRI/um2nc-standalone/pull/74/files#r1721221909