Bump zarr from 2.13.3 to 2.13.6 (#892)

Bumps [zarr](https://github.com/zarr-developers/zarr-python) from 2.13.3 to 2.13.6.
- [Release notes](https://github.com/zarr-developers/zarr-python/releases)
- [Changelog](https://github.com/zarr-developers/zarr-python/blob/main/docs/release.rst)
- [Commits](zarr-developers/zarr-python@v2.13.3...v2.13.6)

---
updated-dependencies:
- dependency-name: zarr
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/requirements-test-optional.txt

@@ -5,4 +5,4 @@ pymongo==4.3.3; implementation_name=='cpython'
 redis==4.4.0
 ruamel.yaml==0.17.21
 tables==3.8.0; implementation_name=='cpython'
-zarr==2.13.3; platform_system!='Windows'
+zarr==2.13.6; platform_system!='Windows'