Drop support for EOL Python 2.6 and 3.3

[hugovk]

They're both EOL and little used.

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

python_version	percent	download_count
2.7	57.65%	235,824
3.6	21.06%	86,156
3.5	14.22%	58,169
3.4	6.18%	25,294
3.3	0.34%	1,382
2.6	0.28%	1,138
3.7	0.26%	1,054
3.2	0.02%	67
3.8	0.00%	3
None	0.00%	1

Source: pypinfo --start-date -41 --end-date -14 --percent --pip --markdown imagesize pyversion

[hugovk]

Stats for July 2018 show 2.6 and 3.3 downloads are now 80% less in 5 months:

python_version	percent	download_count
2.7	49.74%	173,033
3.6	31.19%	108,501
3.5	11.72%	40,785
3.4	4.31%	14,998
3.7	2.85%	9,916
3.3	0.07%	254
2.6	0.07%	241
3.8	0.04%	135
3.2	0.00%	13
None	0.00%	1
Total		347,877

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