[BUG] Cannot serialize a DecimalDtype column

[beckernick]

We can't currently serialize a Decimal64Dtype column. It looks like our serialization procedure expects a str attribute on the Dtype data structure to provide a typestring in line with https://numpy.org/doc/stable/reference/generated/numpy.dtype.html

import cudf
from cudf.core.dtypes import Decimal64Dtype
​
s = cudf.Series([1.31]).astype(Decimal64Dtype(9,2))
s.serialize()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-99-80008fce367e> in <module>
      3 
      4 s = cudf.Series([1.31]).astype(Decimal64Dtype(9,2))
----> 5 s.serialize()

/raid/nicholasb/miniconda3/envs/rapids-gpubdb-20210421/lib/python3.8/site-packages/cudf/core/series.py in serialize(self)
    395         frames.extend(index_frames)
    396         header["index_frame_count"] = len(index_frames)
--> 397         header["column"], column_frames = self._column.serialize()
    398         header["type-serialized"] = pickle.dumps(type(self))
    399         frames.extend(column_frames)

/raid/nicholasb/miniconda3/envs/rapids-gpubdb-20210421/lib/python3.8/site-packages/cudf/core/column/column.py in serialize(self)
   1266         frames = []
   1267         header["type-serialized"] = pickle.dumps(type(self))
-> 1268         header["dtype"] = self.dtype.str
   1269 
   1270         if self.data is not None:

AttributeError: 'Decimal64Dtype' object has no attribute 'str'

conda list | grep "rapids\|dask\|pandas\|arrow\|numpy\|scipy"
# packages in environment at /raid/nicholasb/miniconda3/envs/rapids-gpubdb-20210421:
arrow-cpp                 1.0.1           py38hcb5322d_14_cuda    conda-forge
arrow-cpp-proc            3.0.0                      cuda    conda-forge
cudf                      0.20.0a210421   cuda_11.0_py38_gd501d2c0b9_179    rapidsai-nightly
cuml                      0.20.0a210421   cuda11.0_py38_g2870d59d8_80    rapidsai-nightly
dask                      2021.4.0           pyhd8ed1ab_0    conda-forge
dask-core                 2021.4.0           pyhd8ed1ab_0    conda-forge
dask-cuda                 0.20.0a210421           py38_17    rapidsai-nightly
dask-cudf                 0.20.0a210421   py38_gd501d2c0b9_179    rapidsai-nightly
libcudf                   0.20.0a210421   cuda11.0_gd501d2c0b9_179    rapidsai-nightly
libcuml                   0.20.0a210421   cuda11.0_g2870d59d8_80    rapidsai-nightly
libcumlprims              0.20.0a210408   cuda11.0_g7f19636_2    rapidsai-nightly
librmm                    0.20.0a210421   cuda11.0_g288e8be_17    rapidsai-nightly
numpy                     1.19.5           py38h18fd61f_1    conda-forge
pandas                    1.2.4            py38h1abd341_0    conda-forge
pyarrow                   1.0.1           py38h3e2403a_14_cuda    conda-forge
rmm                       0.20.0a210421   cuda_11.0_py38_g288e8be_17    rapidsai-nightly
scipy                     1.6.2            py38h7b17777_0    conda-forge
ucx                       1.9.0+gcd9efd3       cuda11.0_0    rapidsai-nightly
ucx-proc                  1.0.0                       gpu    rapidsai-nightly
ucx-py                    0.20.0a210419   py38_gcd9efd3_5    rapidsai-nightly

cc @randerzander