pylint & autopep8

[mlin]

YUGE diff introducing autopep8 and pylint to our CI, and cleaning up fuzz discovered in the process. Current pylint score 9.37/10

• make / make test includes pylint in "errors only" mode
• make pretty reformats code with autopep8, and displays pylint report
• Travis build fails if source code isn't at a fixed point for autopep8 (revision pinned in requirements.dev.txt)

[coveralls]

Pull Request Test Coverage Report for Build 139

• 461 of 492 (93.7%) changed or added relevant lines in 12 files are covered.
• 6 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.02%) to 93.721%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
WDL/Type.py	34	35	97.14%
WDL/Value.py	24	25	96.0%
WDL/Error.py	25	27	92.59%
WDL/Walker.py	35	37	94.59%
WDL/Lint.py	20	24	83.33%
WDL/_parser.py	46	53	86.79%
WDL/StdLib.py	71	85	83.53%

Files with Coverage Reduction	New Missed Lines	%
WDL/_parser.py	1	93.56%
WDL/Error.py	1	86.84%
WDL/Type.py	1	97.2%
WDL/StdLib.py	3	85.4%

Totals
Change from base Build 135:	-0.02%
Covered Lines:	1612
Relevant Lines:	1720

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💛 - Coveralls