Unpin dask in documentation environment (#874)

Author: jrbourbeau

ci/environment-docs.yaml

@@ -32,11 +32,7 @@ dependencies:
   - xgboost
   - zict
   - pip
-  # dask 2021.3.0 introduced a regression which causes tests to fail.
-  # The issue has been resolved upstream in dask and will be included
-  # in the next release. We temporarily apply a dask version contraint
-  # to allow CI to pass
-  - dask !=2021.3.0
+  - dask
   - dask-glm
   - dask-xgboost
   - pip: