Drop support for EOL Python 2.6 and 3.3

[hugovk]

This includes PR #588 ("Drop support for EOL Python 2.6") and therefore closes #588 and closes #587 and adds a single new commit to drop 3.3.

Python 3.3 is also EOL and used even less than 2.6.

Here's the pip installs for Flask-SQLAlchemy from PyPI for last month:

python_version	percent	download_count
2.7	83.31%	468,134
3.6	9.90%	55,611
3.5	4.43%	24,867
3.4	2.20%	12,369
2.6	0.07%	420
3.7	0.06%	342
3.3	0.03%	186
None	0.00%	1

Source: pypinfo --start-date -39 --end-date -12 --percent --pip --markdown Flask-SQLAlchemy pyversion

These show the February 2018 download counts for 2.6 and 3.3 are around half what they were in December 2017 (see #588).

[singingwolfboy]

Bump. Can we get this reviewed and merged?