ReferenceFileSystem: use fs.open instead of fs._open

There is a bug in `fsspec==2024.12.0` that causes the `ReferenceFileSystem` to
incorrectly make `fs._open` return a coroutine object instead of a file-like object.
(See a proposed PR to fix this issue: fsspec/filesystem_spec#1769.)

We have a test for the expected behavior (`test_arrow_generator_partitioned` in `tests/unit/lib/test_arrow.py`)
running in the CI environment.
But that does not fail because the latest version of `fsspec` does not get installed in the CI
due to the upper limit set by the `datasets` library.

The `datasets` library is only installed as part of the `hf` and `tests` extras,
so the default installation of `datachain` will encounter this issue.

Fixes #806.

Author: skshetry

src/datachain/lib/arrow.py

@@ -33,7 +33,7 @@ def _open(self, path, mode="rb", *args, **kwargs):
         # reads the whole file in-memory.
         (uri,) = self.references[path]
         protocol, _ = split_protocol(uri)
-        return self.fss[protocol]._open(uri, mode, *args, **kwargs)
+        return self.fss[protocol].open(uri, mode, *args, **kwargs)
 
 
 class ArrowGenerator(Generator):