Be more explicit about dask dependencies

Author: marcharper

requirements.txt

@@ -1,3 +1,8 @@
+# dask dependencies: https://docs.dask.org/en/latest/install.html
+cloudpickle>=0.2.2
+fsspec>=0.6.0
+toolz>=0.8.2
+
 dask>=2.9.2
 hypothesis==5.19.3
 matplotlib>=3.0.3
@@ -6,6 +11,4 @@ pandas>=1.0.0
 prompt-toolkit>=3.0
 pyyaml>=3.01
 scipy>=1.3.3
-# toolz required for dask
-toolz>=0.10.0
 tqdm>=4.39.0