fixing rolling_window issue with cupy

[negin513]

• Closes xarray.rolling_window Converts dims Argument from Tuple to List Causing Issues for Cupy-Xarray #7890.
• User visible changes (including notable bug fixes) are documented in whats-new.rst

• Relevant Cupy PR: Add cupy.lib.stride_tricks.sliding_window_view cupy/cupy#7575

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[negin513]

This PR fix a part of the issue we see when using rolling_window with cupy.
While numpy.normalize_axis_tuple can handle lists, therefore not complaining when using numpy-backed xarray objects, its cupy equivalent (cupy.core.internal._normalize_axis_indices) does not accept lists and only accept tuples. This PR change the data type for dims to tuple so we can make rolling_window to work for both numpy and cupy.

[dcherian]

Thanks @negin513 ! LGTM. Welcome to Xarray!

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