Update Python versions: add 3.6, remove EOL

[hugovk]

Fixes #56.

Python 3.2 is causing build problems (#56) and it's been decided it's easiest to drop it as it's EOL and no longer receiving security updates.

Now is also a good time to add Python 3.6 and remove the other EOL versions (2.5, 2.6, 3.2 and 3.3), which allows the code to be modernised.

Here's the pip installs for holidays from PyPI for the last month:

$ pypinfo --percent --pip holidays pyversion
python_version percent download_count
-------------- ------- --------------
2.7              70.7%         11,946
3.6              19.7%          3,332
3.5               7.3%          1,239
3.4               2.2%            376
2.6               0.0%              3
3.3               0.0%              1

Those versions are barely used if at all.

[hugovk]

Note the build is failing due to #77 but here's a passing build with flake8 disabled:
https://travis-ci.org/hugovk/python-holidays/builds/294470197

I've confirmed locally no new flake8 issues are introduced.

[coveralls]

Coverage remained the same at 98.982% when pulling a05ea4a on hugovk:update-versions into 988bdf8 on ryanss:master.

[hugovk]

I've tacked #80 onto this and now the build passes.

[dr-prodigy]

Superseded by 1452cc3 : please refresh your repo and re-apply small fixes not related to python version. Thx!

[hugovk]

@dr-prodigy The main point of this PR is to remove those Python versions which are EOL (2.5, 2.6, 3.2 and 3.3), and then update the code to ease the maintenance burden and make use of more modern Python.

Here's updated pip installs for holidays from PyPI for the last month (via pypinfo --percent --pip --markdown holidays pyversion):

python_version	percent	download_count
2.7	76.0%	16,846
3.6	18.9%	4,194
3.5	3.6%	796
3.4	1.5%	332
2.6	0.0%	2
3.7	0.0%	1

Of the EOL versions, this shows only two installs for 2.6, and nothing for 2.5, 3.2 and 3.3.

[hugovk]

See PR #92!