STYLE ban np.testing
xref https://github.com/pandas-dev/pandas/pull/43674#discussion_r720831806

There used to be a linting rule for this, but I missed it when moving linting rules over to `pandas-dev-flaker` in https://github.com/pandas-dev/pandas/pull/40906 - sorry about that, I thought I'd checked that everything had been preserved

Would be good to use this one from `pandas-dev-flaker` directly, but without a `--per-file-selects` option it'd be rather cumbersome (https://github.com/PyCQA/flake8/issues/344#issuecomment-940054283)