Drop unsupported Python 3.3

[hugovk]

Issue #47 says:

Ansible (one of our largest downstream users on Python versions older than 2.6) is officially making all code only support 2.6 or newer in April 2017.

Well, now Ansible say:

Note
Ansible supports Python version 3.5 and above only.

https://docs.ansible.com/ansible/latest/python_3_support.html

So Ansible no longer support Python 3.3 either. Coupled with the fact Python 3.3 has been EOL since last month, means it can be dropped. Here are more reasons:

• pip 10 will deprecate Python 3.3 support, pip 11 won't support it (Deprecate/Drop Support for Python 3.3? pypa/pip#3796)
• Very little PyPI traffic (June 2016) Deprecate/Drop Support for Python 3.3? pypa/pip#3796

(I also note that setup.py wasn't claiming Python 3.3 support, but perhaps that was an oversight when adding the classifiers in 16bd7d0.)

[hugovk]

The CI failure is an unrelated problem on Python 2.6 that also affects latest master builds. Will be fixed by #83.

[hugovk]

Here's the pip installs for pyyaml from PyPI for the last month -- only 0.1% for 3.3:

$ pypinfo --percent --pip pyyaml pyversion
python_version percent download_count
-------------- ------- --------------
2.7              89.4%     11,732,683
3.5               3.6%        470,139
3.6               3.3%        430,133
3.4               2.7%        351,614
2.6               1.0%        127,184
3.3               0.1%          7,931
3.7               0.0%          3,638
3.2               0.0%            284
None              0.0%            126
3.1               0.0%              2
2.8               0.0%              2

[hugovk]

Updated to resolve conflict and the CI is now green.

Here's the pip installs for pyyaml from PyPI for last month, Python 3.3 use has dropped by 2/3 since October:

python_version	percent	download_count
2.7	84.34%	8,225,438
3.6	7.99%	779,730
3.5	4.87%	474,696
3.4	2.32%	226,614
2.6	0.41%	40,270
3.7	0.03%	3,239
3.3	0.03%	2,641
3.2	0.00%	114
None	0.00%	13
3.8	0.00%	12

Source: pypinfo --start-date -37 --end-date -8 --percent --pip --markdown pyyaml pyversion

[ingydotnet]

Applied. Pushed. Thanks.

This will go into the upcoming 4.01 release.

See: https://github.com/yaml/pyyaml/projects/1