Fix import format pandas/tests/indexing
- [x] xref #23334
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Ran `isort --recursive pandas` and then checked imports using `isort --recursive --check-only pandas`

PR capped at 20 files, but 21 files changed because it includes setup.cfg which has also been changed.
