Use cuDF Frame instead of Table

[madsbk]

In v21.12, cuDF removed Table in favor of Frame as base class, which is now part of the office API.

We now import cudf.core.frame.Frame if cudf._lib.table.Table isn't available.
Also, this PR adds some # type: ignore to make mypy pass.

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[madsbk]

Notice, the CI error is because of #746 and will be fixed when dask/distributed#5380 has been merged.

15:17:34 Traceback (most recent call last):
15:17:34   File "/opt/conda/envs/rapids/lib/python3.7/site-packages/distributed/utils.py", line 648, in log_errors
15:17:34     yield
15:17:34   File "/opt/conda/envs/rapids/lib/python3.7/site-packages/cudf/comm/serialize.py", line 22, in cuda_deserialize_cudf_object
15:17:34     return Serializable.device_deserialize(header, frames)
15:17:34   File "/opt/conda/envs/rapids/lib/python3.7/site-packages/cudf/core/abc.py", line 142, in device_deserialize
15:17:34     obj = typ.deserialize(header, frames)
15:17:34   File "/opt/conda/envs/rapids/lib/python3.7/site-packages/cudf/core/dataframe.py", line 546, in deserialize
15:17:34     header, frames[header["index_frame_count"] :]
15:17:34   File "/opt/conda/envs/rapids/lib/python3.7/site-packages/cudf/core/frame.py", line 128, in deserialize
15:17:34     cls_deserialize = pickle.loads(header["type-serialized"])
15:17:34 ValueError: unsupported pickle protocol: 5

[quasiben]

Thanks for updating. We'll merge when the Dask PR is merged in