Bump xarray from 0.18.1 to 0.18.2

Bumps [xarray](https://github.com/pydata/xarray) from 0.18.1 to 0.18.2.
- [Release notes](https://github.com/pydata/xarray/releases)
- [Changelog](https://github.com/pydata/xarray/blob/master/HOW_TO_RELEASE.md)
- [Commits](pydata/xarray@v0.18.1...v0.18.2)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs_requirements.txt

@@ -84,5 +84,5 @@ wcwidth==0.2.5
 webencodings==0.5.1
 widgetsnbextension==3.5.1
 wincertstore==0.2
-xarray==0.18.1
+xarray==0.18.2
 zipp==3.4.1

requirements.txt

@@ -90,7 +90,7 @@ websockets~=9.0
 widgetsnbextension~=3.5.1
 wincertstore~=0.2
 wrapt~=1.12.1
-xarray~=0.18.1
+xarray~=0.18.2
 zipp~=3.4.1
 # not strict requirements but needed for the legacy pyqtgraph based plotting
 PyQt5<5.16