Drop support for EOL Python 2.6, add support for 3.6

[hugovk]

Fixes #426.

[codecov-io]

Codecov Report

Merging #471 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #471   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          14     13    -1     
  Lines        3385   3372   -13     
  Branches      235    235           
=====================================
- Hits         3385   3372   -13

Impacted Files	Coverage Δ
arrow/util.py	100% <ø> (ø)	⬆️
arrow/parser.py	100% <100%> (ø)	⬆️
arrow/arrow.py	100% <100%> (ø)	⬆️
tests/arrow_tests.py	100% <100%> (ø)	⬆️
arrow/factory.py	100% <100%> (ø)	⬆️

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[ramonsaraiva]

I'm not entirely sure if we should drop the usage of indexes in format for every single case. What do you guys think @Syeberman @andrewelkins ?

[hugovk]

Rebased on master to resolve conflicts.

Here's the pip installs for arrow from PyPI for April 2018:

python_version	percent	download_count
3.6	71.02%	629,512
2.7	22.49%	199,330
3.5	4.04%	35,802
3.4	2.35%	20,846
3.7	0.04%	376
2.6	0.04%	346
3.3	0.01%	117
3.8	0.00%	2
Total		886,331

Source: pypinfo --start-date 2018-04-01 --end-date 2018-04-30 --percent --pip --markdown arrow pyversion

[hugovk]

Closing in favour of PR #472.