Drop support for Python 2.6 and 3.2

[JonathonReinhart]

Closes #118

[codecov-io]

Codecov Report

Merging #119 into master will decrease coverage by <.01%.
The diff coverage is 77.14%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   94.95%   94.95%   -0.01%     
==========================================
  Files           9        9              
  Lines         595      575      -20     
==========================================
- Hits          565      546      -19     
+ Misses         30       29       -1

Impacted Files	Coverage Δ
scuba/utils.py	100% <ø> (ø)	⬆️
scuba/compat.py	100% <ø> (+3.84%)	⬆️
scuba/dockerutil.py	89.06% <33.33%> (ø)	⬆️
scuba/version.py	68.42% <66.66%> (-0.81%)	⬇️
scuba/config.py	98.76% <83.33%> (ø)	⬆️
scuba/__main__.py	96.72% <92.85%> (ø)	⬆️

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[JonathonReinhart]

FYI: This branch will need updated shortly. I want to complete #88 first, and release Scuba v2.3.0 before merging this PR in preparation for v3.0.0.