Latest version of seaborn removed six

The 0.10.0 release of seaborn dropped python 2 support after it reached end-of-life at the end of last year. As part of this, it looks like they removed internal uses of six. This is causing failures in the violin plots, see trace below.

If pesummary plans to continue to support python 2 we can switch to using the free-standing six package, https://six.readthedocs.io/. Alternatively, we could also discontinue python 2 support and remove all six and __future__ imports.

@charlie.hoy do you have a preferred method? I'm happy to implement the change.

 04:08 bilby_pipe INFO    : Running command: /opt/conda/envs/python37/bin/summarypages --webdir test_outdir/results_page --email albert.einstein@ligo.org --config test_pesummary.ini --samples test_outdir/result/label_data0_0_analysis_H1L1_dynesty_result.json -a IMRPhenomPv2 --gwdata test_outdir/data/label_data0_0_generation_data_dump.pickle
 Traceback (most recent call last):
   File "/opt/conda/envs/python37/bin/summarypages", line 10, in <module>
     sys.exit(main())
   File "/opt/conda/envs/python37/lib/python3.7/site-packages/pesummary/cli/summarypages.py", line 190, in main
     from .summaryplots import PlotGeneration
   File "/opt/conda/envs/python37/lib/python3.7/site-packages/pesummary/cli/summaryplots.py", line 23, in <module>
     from pesummary.gw.plots.main import _PlotGeneration
   File "/opt/conda/envs/python37/lib/python3.7/site-packages/pesummary/gw/plots/main.py", line 31, in <module>
     from pesummary.gw.plots import publication
   File "/opt/conda/envs/python37/lib/python3.7/site-packages/pesummary/gw/plots/publication.py", line 22, in <module>
     from pesummary.gw.plots import violin
   File "/opt/conda/envs/python37/lib/python3.7/site-packages/pesummary/gw/plots/violin.py", line 30, in <module>
     from seaborn.external.six import string_types
 ModuleNotFoundError: No module named 'seaborn.external.six'

Related bilby_pipe issue bilby_pipe#155 (closed)

UPDATE:

Six appears to be used three times, once from six and twice from seaborn.external.six

pesummary/core/plots/kde.py:from six import string_types
pesummary/gw/plots/violin.py:from seaborn.external.six import string_types
pesummary/gw/plots/violin.py:from seaborn.external.six.moves import range

There also only seems to be one __future__ import

pesummary/gw/plots/violin.py:from __future__ import division

If we remove python 2 support, we can just test that something is a str rather than using string_types.