Update xarray requirement from ~=0.12.1 to ~=0.18.0

[dependabot-preview[bot]]

Updates the requirements on xarray to permit the latest version.

Release notes

Sourced from xarray's releases.

v0.18.0

This release brings a few important performance improvements, a wide range of usability upgrades, lots of bug fixes, and some new features. These include a plugin API to add backend engines, a new theme for the documentation, curve fitting methods, and several new plotting functions.

Commits

• 8b028e3 Release-workflow: Bug fix (#5273)
• 01c1d25 more maintenance on whats-new.rst (#5272)
• eec2b59 v0.18.0 release highlights (#5266)
• d63c26c Fix exception when display_expand_data=False for file-backed array. (#5235)
• 477fc2f Warn ignored keep attrs (#5265)
• 60e3963 Disable workflows on forks (#5267)
• 054f81a fix the built wheel test (#5270)
• 10648cb pypi upload workflow maintenance (#5269)
• f5e4fd5 Add GitHub action for publishing artifacts to PyPI (#5244)
• c1aea94 Parametrize test for setitem (for dask array) (#5243)
• Additional commits viewable in compare view

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[dependabot-preview[bot]]

Superseded by #125.