return type of fs.cat

[d-v-b]

fsmap.getitems assumes that fs.cat returns a dict: see https://github.com/intake/filesystem_spec/blob/master/fsspec/mapping.py#L95. However, there is a branch in fs.cat that returns bytes, not a dict: see https://github.com/intake/filesystem_spec/blob/master/fsspec/spec.py#L728. Is this deliberate? Should fsmap.getitems convert the non-dict value to a dict, or should fs.cat always return a dict (as per its docstring)?

And maybe some type annotations would be helpful here.

(This came up while testing zarr against the latest fsspec, which causes a test failure because of this API issue)

[d-v-b]

see #710