Drop support for EOL Python 2.6 and 3.3

[hugovk]

Further to #455, I'd like to suggest dropping support for Python 2.6 and 3.3. Both are EOL (2.6 for four years), meaning they no longer receive updates, including security updates.

Removing them will help ease the maintenance burden, and allow to use more modern features of Python. Travis CI will also quicker, from ~23 min run time (total time ~1 hr 4 min) to ~11 min run time (total time ~43 min).

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

See also:

• https://snarky.ca/stop-using-python-2-6/
• http://www.curiousefficiency.org/posts/2015/04/stop-supporting-python26.html
• http://www.python3statement.org

They're also little used. Here's the pip installs for babel from PyPI for the last month (via pypinfo --percent --pip --markdown babel pyversion).

python_version	percent	download_count
2.7	75.2%	965,130
3.6	10.8%	139,110
3.5	8.1%	103,400
3.4	5.4%	69,261
2.6	0.2%	2,948
3.3	0.2%	2,069
3.7	0.1%	1,865
3.2	0.0%	55
None	0.0%	17

This PR also adds python_requires to help pip and does some minor tidy ups.

[codecov-io]

Codecov Report

Merging #546 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #546      +/-   ##
==========================================
- Coverage   90.06%   90.05%   -0.01%     
==========================================
  Files          24       24              
  Lines        4046     4044       -2     
==========================================
- Hits         3644     3642       -2     
  Misses        402      402

Impacted Files	Coverage Δ
babel/numbers.py	98% <ø> (-0.02%)	⬇️
babel/messages/checkers.py	64.63% <ø> (ø)	⬆️
babel/plural.py	95.68% <100%> (ø)	⬆️
babel/dates.py	91.24% <100%> (ø)	⬆️
babel/languages.py	100% <100%> (ø)	⬆️
babel/messages/frontend.py	86.25% <100%> (ø)	⬆️
babel/messages/catalog.py	94.62% <100%> (ø)	⬆️

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[akx]

This seems alright. Thank you @hugovk :)