Issue with pyarrow 14.0.1

See https://github.com/huggingface/datasets-server/pull/2089 for reference

```
from datasets import (Array2D, Dataset, Features)
feature_type = Array2D(shape=(2, 2), dtype="float32")
content = [[0.0, 0.0], [0.0, 0.0]]
features = Features({"col": feature_type})
dataset = Dataset.from_dict({"col": [content]}, features=features)
```

generates

```
/home/slesage/hf/datasets-server/libs/libcommon/.venv/lib/python3.9/site-packages/datasets/features/features.py:648: FutureWarning: pyarrow.PyExtensionType is deprecated and will refuse deserialization by default. Instead, please derive from pyarrow.ExtensionType and implement your own serialization mechanism.
  pa.PyExtensionType.__init__(self, self.storage_dtype)
/home/slesage/hf/datasets-server/libs/libcommon/.venv/lib/python3.9/site-packages/datasets/features/features.py:1661: RuntimeWarning: pickle-based deserialization of pyarrow.PyExtensionType subclasses is disabled by default; if you only ingest trusted data files, you may re-enable this using `pyarrow.PyExtensionType.set_auto_load(True)`.
In the future, Python-defined extension subclasses should derive from pyarrow.ExtensionType (not pyarrow.PyExtensionType) and implement their own serialization mechanism.

  obj = {field.name: generate_from_arrow_type(field.type) for field in pa_schema}
/home/slesage/hf/datasets-server/libs/libcommon/.venv/lib/python3.9/site-packages/datasets/features/features.py:1661: FutureWarning: pyarrow.PyExtensionType is deprecated and will refuse deserialization by default. Instead, please derive from pyarrow.ExtensionType and implement your own serialization mechanism.
  obj = {field.name: generate_from_arrow_type(field.type) for field in pa_schema}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/slesage/hf/datasets-server/libs/libcommon/.venv/lib/python3.9/site-packages/datasets/arrow_dataset.py", line 924, in from_dict
    return cls(pa_table, info=info, split=split)
  File "/home/slesage/hf/datasets-server/libs/libcommon/.venv/lib/python3.9/site-packages/datasets/arrow_dataset.py", line 693, in __init__
    inferred_features = Features.from_arrow_schema(arrow_table.schema)
  File "/home/slesage/hf/datasets-server/libs/libcommon/.venv/lib/python3.9/site-packages/datasets/features/features.py", line 1661, in from_arrow_schema
    obj = {field.name: generate_from_arrow_type(field.type) for field in pa_schema}
  File "/home/slesage/hf/datasets-server/libs/libcommon/.venv/lib/python3.9/site-packages/datasets/features/features.py", line 1661, in <dictcomp>
    obj = {field.name: generate_from_arrow_type(field.type) for field in pa_schema}
  File "/home/slesage/hf/datasets-server/libs/libcommon/.venv/lib/python3.9/site-packages/datasets/features/features.py", line 1381, in generate_from_arrow_type
    return Value(dtype=_arrow_to_datasets_dtype(pa_type))
  File "/home/slesage/hf/datasets-server/libs/libcommon/.venv/lib/python3.9/site-packages/datasets/features/features.py", line 111, in _arrow_to_datasets_dtype
    raise ValueError(f"Arrow type {arrow_type} does not have a datasets dtype equivalent.")
ValueError: Arrow type extension<arrow.py_extension_type<pyarrow.lib.UnknownExtensionType>> does not have a datasets dtype equivalent.
```