Drop support for EOL Python

[hugovk]

Python 2.6 and 3.3 are EOL (since 2013-10-29 and 2017-09-29 respectively) and no longer receiving security updates (or any updates) from the core Python team.

They're also failing the CI and little used.

Here's the pip installs for lazy-object-proxy from PyPI for August 2018:

python_version	percent	download_count
3.6	41.63%	916,939
2.7	34.56%	761,109
3.5	11.28%	248,483
3.7	9.22%	203,077
3.4	3.23%	71,081
3.3	0.04%	792
2.6	0.02%	495
3.8	0.02%	390
3.2	0.00%	44
None	0.00%	1
Total		2,202,411

Source: pypinfo --start-date 2018-08-01 --end-date 2018-08-31 --json lazy-object-proxy pyversion

This PR:

• Drops support for EOL Python 2.6 and 3.3
• Upgrades Python syntax with pyupgrade https://github.com/asottile/pyupgrade
• Add python_requires to setup.py to help pip install the right package version
• Remove pytest-capturelog dependency, as it's been merged into pytest's core

There's still some Coverage-related errors on the CI, but this improves things:

Travis CI before

https://travis-ci.org/hugovk/python-lazy-object-proxy/builds/427327283

Travis CI after

https://travis-ci.org/hugovk/python-lazy-object-proxy/builds/427371253

AppVeyor before

https://ci.appveyor.com/project/hugovk/python-lazy-object-proxy/build/master-3

AppVeyor after

https://ci.appveyor.com/project/hugovk/python-lazy-object-proxy/build/rm-eol-6

[codecov-io]

Codecov Report

Merging #24 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #24   +/-   ##
=======================================
  Coverage   94.06%   94.06%           
=======================================
  Files           7        7           
  Lines        2022     2022           
  Branches      282      282           
=======================================
  Hits         1902     1902           
  Misses         97       97           
  Partials       23       23

Impacted Files	Coverage Δ
tests/test_lazy_object_proxy.py	92.55% <ø> (ø)	⬆️
src/lazy_object_proxy/slots.py	96.37% <100%> (ø)	⬆️
src/lazy_object_proxy/simple.py	97.45% <100%> (ø)	⬆️

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[hugovk]

Rebased on master.