Remove non-existent import.

[brittonsmith]

After merging PR #2163 in yt, the six package no longer exists. Since we're not supporting python 2 in this branch anyway, this is fine. This should get testing working again.

[brittonsmith]

There are some other issues I'll need to fix, but we'll have to wait on this PR.

[coveralls]

Pull Request Test Coverage Report for Build 863

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 126 unchanged lines in 7 files lost coverage.
• Overall coverage decreased (-0.08%) to 71.989%

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Files with Coverage Reduction	New Missed Lines	%
/home/travis/build/trident-project/trident/trident/line_database.py	9	90.52%
/home/travis/build/trident-project/trident/trident/ray_generator.py	10	88.0%
/home/travis/build/trident-project/trident/trident/light_ray.py	11	87.71%
/home/travis/build/trident-project/trident/trident/ion_balance.py	18	84.71%
/home/travis/build/trident-project/trident/trident/config.py	25	51.11%
/home/travis/build/trident-project/trident/trident/absorption_spectrum/absorption_spectrum.py	26	88.14%
/home/travis/build/trident-project/trident/trident/utilities.py	27	36.43%

Totals
Change from base Build 706:	-0.08%
Covered Lines:	1524
Relevant Lines:	2117

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💛 - Coveralls