Fix setup.py and use of requirements files

[jeking3]

There were requirements scattered about in both setup.py and requirements.txt - this consolidated them all into requirements files. It also adds python 3.5 and 3.6 to the tests.

[codecov-io]

Codecov Report

Merging #853 into master will not change coverage.
The diff coverage is 100%.

@@ Coverage Diff @@## master #853 +/- ##
=======================================
Coverage 94.79% 94.79% =======================================
Files 59 59 Lines 9603 9603 =======================================
Hits 9103 9103 Misses 500 500

Impacted Files	Coverage Δ
git/cmd.py	84% <100%> (ø)	⬆️

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[jeking3]

@Byron is it important to maintain support for Python 3.4? It doesn't understand some of the setup.py changes. I could mark the project as requiring Python 3.5 or later. This PR adds 3.6 and 3.7 testing to appveyor and it consolidates all the requirements logic for setup. Next PR will set some of the currently failing appveyor tests as allowed failures (miniconda variants). Thanks.