fix np deprecation warnings for non-tuple sequence in multi-dim indexing

[neutrinoceros]

PR Summary

in recent versions of numpy, this pattern is deprecated:

# let arr be a 3D array
arr[[slice(None) for _ in range(3)]]

and one gets this warning:

FutureWarning: Using a non-tuple sequence for multidimensional indexing is deprecated; use `arr[tuple(seq)]` instead of `arr[seq]`. In the future this will be interpreted as an array index, `arr[np.array(seq)]`, which will result either in an error or a different

I discovered the places this pattern was used in yt by checking the logs on Travis.

Note that this is not a bug yet, but I'm still using the label since it will become an error.

PR Checklist

• pass black --check yt/
• pass isort . --check --diff
• pass flake8 yt/
• pass flynt yt/ --fail-on-change --dry-run -e yt/extern