Use new fsspec

Author: Martin Durant

requirements_dev_optional.txt

@@ -19,6 +19,6 @@ pytest-doctestplus==0.4.0
 pytest-remotedata==0.3.2
 h5py==2.10.0
 s3fs==0.5.1; python_version > '3.6'
-git+https://github.com/intake/filesystem_spec; python_version > '3.6'
+fsspec==0.8.3 python_version > '3.6'
 moto>=1.3.14; python_version > '3.6'
 flask