Drop support for EOL Python versions

[hugovk]

Python 2.6, 3.3 and 3.4 are EOL and no longer receiving security updates (or any updates) from the core Python team.

Version	Release date	Supported until
2.6	2008-10-01	2013-10-29
3.3	2012-09-29	2017-09-29
3.4	2014-03-16	2019-03-16

Source: https://en.wikipedia.org/wiki/CPython#Version_history

They're also little used. Here's the pip installs for google-apitools from PyPI for March 2019:

category	percent	downloads
2.7	89.28%	663,944
3.6	9.54%	70,984
3.5	0.57%	4,217
3.7	0.29%	2,144
null	0.17%	1,239
2.6	0.14%	1,078
3.4	0.01%	75
3.8	0.00%	1
Total		743,682

Source: pypistats python_minor google-apitools --last-month # pip install pypistats

This PR also:

• adds python_requires to help pip install the right version for the running Python
• adds Trove classifiers to make supported versions clear on PyPI
• Remove unittest2 dependency, which was required for EOL Python 2.6
• sudo is no longer needed on Travis CI

[coveralls]

Coverage increased (+0.008%) to 91.718% when pulling 7c0966f on hugovk:update-versions into 6d64cb6 on google:master.

[tvalentyn]

Did you by chance try apitools with Python 3.6, 3.7, @hugovk?

[hugovk]

I just had a quick go, but it failed so I think that can be dealt with separately to this:

https://travis-ci.org/hugovk/apitools/builds/530685178

[cclauss]

What is the status of this one?

[hugovk]

Merge conflict resolved. EOL numbers are going down:

category	percent	downloads
2.7	77.18%	751,697
3.6	16.24%	158,180
3.5	4.06%	39,524
3.7	2.35%	22,868
null	0.10%	950
2.6	0.07%	724
3.4	0.01%	61
3.8	0.00%	1
Total		974,005

Date range: 2019-08-01 - 2019-08-31

[cclauss]

% pypistats python_minor google-apitools --last-month

category	percent	downloads
2.7	58.78%	724,425
3.5	19.88%	245,064
3.7	11.20%	137,983
3.6	9.92%	122,317
3.8	0.17%	2,059
null	0.05%	589
2.6	0.00%	20
3.4	0.00%	7
3.3	0.00%	1
Total		1,232,465

Date range: 2020-03-01 - 2020-03-31