install optional fsspec backends in the CI for some python versions

Author: lobis

.github/workflows/build-test.yml

@@ -51,6 +51,10 @@ jobs:
           mamba install xrootd
           conda list
 
+      - name: "Install fsspec backends"
+        if: "matrix.python-version == 3.11"
+        run: python -m pip install s3fs
+
       - name: "Pip install the package"
         run: python -m pip install .[test,dev]