CHORE: Change `test_plotting.py` to not use `npt.NDArray` as argument in `check()`

[Dr-Irv]

With the changes we've made to how we refer to numpy types, we need to change lines like this in test_plotting.py:

check(assert_type(df.hist(), npt.NDArray[np.object_]), npt.NDArray[np.object_])

to

check(assert_type(df.hist(), npt.NDArray[np.object_]), np.ndarray)

See numpy/numpy#30319 (comment) and comments below for discussion. We shouldn't be passing the typing type npt.NDArray as the klass to check()