Drop support for EOL Python 2.6 and 3.3

[hugovk]

Here's the pip installs for coinbase from PyPI for the last month (via pypinfo --percent --pip coinbase pyversion):

python_version	percent	download_count
2.7	55.1%	1,150
3.6	26.6%	555
3.5	14.1%	294
3.4	3.4%	70
3.7	0.7%	14
2.6	0.1%	3

Python 2.6 and 3.3 are EOL and no longer receives security updates. Both are little used, if at all.

This also removes the outdated .python-version file, which has probably not been used for a long time. Let me know if you want to keep it/update it.

[sds]

Thanks a ton, @hugovk!