Drop support for EOL Python 3.3

[hugovk]

Python 3.3 is EOL (since 2017-09-29) 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 dominate from PyPI for August 2018:

python_version	percent	download_count
3.6	45.11%	26,193
2.7	27.62%	16,039
3.5	14.32%	8,315
3.7	7.63%	4,431
3.4	4.95%	2,872
3.3	0.28%	161
2.6	0.09%	51
Total		58,062

Source: pypinfo --start-date 2018-08-01 --end-date 2018-08-31 --markdown --percent dominate pyversion

It is also failing the build (eg. see PR #100 which only changed the README) because dependencies no longer support Python 3.3.

This PR also upgrades pip which fixes the Python 3.4 build.

[coveralls]

Coverage remained the same at 96.661% when pulling aced523 on hugovk:rm-3.3 into a23defe on Knio:master.