Bump xarray from 0.16.2 to 0.19.0 in /extra_requirements (#615)

* Bump xarray from 0.16.2 to 0.19.0 in /extra_requirements

Bumps [xarray](https://github.com/pydata/xarray) from 0.16.2 to 0.19.0.
- [Release notes](https://github.com/pydata/xarray/releases)
- [Changelog](https://github.com/pydata/xarray/blob/main/HOW_TO_RELEASE.md)
- [Commits](pydata/xarray@v0.16.2...v0.19.0)

---
updated-dependencies:
- dependency-name: xarray
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

* Update requirements-tests.txt

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Joran Angevaare <jorana@nikhef.nl>

extra_requirements/requirements-tests.txt

@@ -35,6 +35,9 @@ tensorflow~=2.5.0; python_version=="3.6"
 tensorflow~=2.4; python_version>="3.7"
 
 tqdm==4.62.0
-xarray==0.16.2
+
+xarray==0.19.0; python_version>="3.7"
+xarray==0.16.2; python_version=="3.6"
+
 utilix==0.6.1
 zstd==1.5.0.2                                   # Strax dependency