Drop support for Python 2.6.
Drop support for Python 2.6.

It's broken because pip and setuptools have dropped support, and
no-one seems to be bothered or care about fixing it, so let's do this.

Closes #126

manylinux/manylinux,manylinux/manylinux,Parsely/manylinux,pypa/manylinux,pypa/manylinux,pypa/manylinux,Parsely/manylinux