Drop support for EOL Python 3.3

[hugovk]

Python 3.3 is EOL and no longer receiving security updates (or any updates) from the core Python team.

It's also little used. Here's the pip installs for lxml from PyPI for July 2018:

python_version	percent	download_count
2.7	59.75%	637,606
3.6	22.04%	235,237
3.5	11.17%	119,194
3.4	4.08%	43,500
3.7	2.79%	29,729
2.6	0.10%	1,061
3.3	0.05%	553
3.8	0.01%	160
3.2	0.00%	41
None	0.00%	5
Total		1,067,086

Source: pypinfo --start-date 2018-07-01 --end-date 2018-07-31 --percent --markdown lxml pyversion

There's not much code to change, but one benefit is to drop a couple of builds from the CI, and one less thing to worry about.

[hugovk]

@Heartcore11 Please don't spam old PRs and issues.