DOC: Fix isort output in CI
In the CI, we're using `isort` to check whether the imports in Python files are sorted in an standard way. See https://github.com/pandas-dev/pandas/blob/master/ci/code_checks.sh#L114

The way it is implemented, we expect isort to not print anything in the terminal if everything is ok, and to show errors for the errors encountered. This is mostly true, except that `isort` is also reporting when there are files being skipped.

The result of this, is that we highlight in the CI as an error when there are files being skipped. See https://github.com/pandas-dev/pandas/pull/30946/checks?check_run_id=385833706#step:6:36

This can be misleading, and we should try to avoid it. Without adding much complexity to the already complex CI. Not sure if there is an `isort` parameter to simply not report the skipped files. Or if there is an easy way to not highlight the skipped files as an error.
