Remove setup.py test command in favor of pytest and tox

[jdufresne]

Using pytest and tox directly is simpler and more conventional then
going through setup.py. Using setup.py installs packages as eggs where
as tox uses the more typical pip.

Overall simplifies setup.py.

Fixes #415

[coveralls]

Pull Request Test Coverage Report for Build 76

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.565%

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Totals
Change from base Build 75:	0.0%
Covered Lines:	2143
Relevant Lines:	2874

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 76

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.7%) to 73.84%

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Totals
Change from base Build 75:	-0.7%
Covered Lines:	1592
Relevant Lines:	2156

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💛 - Coveralls

[jpadilla]

Perfect, thanks! 🎆