Remove dependency on six.

[bdice]

I noticed while working on #9448 that we import six but it doesn't appear to be a dependency of cuDF. Since that package is meant for Python 2 compatibility and we do not support Python 2, I think it can be removed.

six is probably being installed as a transitive dependency of some dependency of cuDF, if I were to guess, otherwise I'd think tests would fail.

Note: the queryutils.py module containing these lines may be somewhat outdated. The module is not covered by tests from 21.08 through 21.12, but was covered by tests in 21.06. It is currently only used in a couple places, which may deserve some cleanup: https://github.com/rapidsai/cudf/search?q=queryutils

[vyasr]

queryutils is definitely outdated. However, it is used in the implementation of df.query. I considered removing and replacing it via AST evaluation, but that effort was put on hold for a while due to the substantial refactorings of the AST internals undertaken during 21.10. Once I can assess that #8022 offers a suitable replacement for queryutils (no regressions with respect to the various features pandas expects df.query to have) queryutils should be removable.

[codecov]

Codecov Report

Merging #9495 (709762e) into branch-21.12 (ab4bfaa) will decrease coverage by 0.12%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-21.12    #9495      +/-   ##
================================================
- Coverage         10.79%   10.66%   -0.13%     
================================================
  Files               116      117       +1     
  Lines             18869    19727     +858     
================================================
+ Hits               2036     2104      +68     
- Misses            16833    17623     +790     

Impacted Files	Coverage Δ
python/dask_cudf/dask_cudf/sorting.py	92.90% <0.00%> (-1.21%)	⬇️
python/cudf/cudf/io/csv.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/hdf.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/orc.py	0.00% <0.00%> (ø)
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/_version.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/abc.py	0.00% <0.00%> (ø)
python/cudf/cudf/api/types.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/dlpack.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
... and 66 more

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[bdice]

The test failure will be resolved once #9508 is merged and CI is re-triggered.

[bdice]

@gpucibot rerun tests

[bdice]

@gpucibot rerun tests

[bdice]

Tests have failed on random download errors twice. I am re-running again. This is ready to merge once CI gets it together. 🙃

[galipremsagar]

rerun tests

[vyasr]

@bdice for future reference, the request to rerun test does not go through gpucibot.

[galipremsagar]

@gpucibot merge