Rely on Python 3.6 and Python 2.7

[martinholmer]

This pull request simplifies the Tax-Calculator check-in testing and conda-package releasing processes by relying only on Python 2.7 and Python 3.6, which are the only two versions of Python available on the Anaconda download page. There have been only a few (mostly linux) downloads of Python 3.5 taxcalc packages out of a total of 4,367 downloads, and Python 3.6 is fairly mature now. There seems to be no need to build the five taxcalc packages for Python 3.5.

[codecov-io]

Codecov Report

Merging #1587 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1587   +/-   ##
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  Coverage     100%    100%           
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  Files          37      37           
  Lines        2743    2743           
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  Hits         2743    2743

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[martinholmer]

The need to eliminate the exact pinning of the conda package in the appveyor.yml file in pull request #1587 is yet another example of the evils of exact pinning discussed in PolicyBrain issue 697.

The exact pinning of the conda package to version 4.2.13 created a package inconsistency with conda==4.2.13 requiring old versions of packages that are no longer supported by Python 3.6.
This inconsistency was eliminated by the use of at-least pinning conda>=4.3.0. Notice that the current version of conda is 4.3.29.