Require six >= 1.11.0

[avdv]

It is not good practice pinning versions in install_requires, it should only define
minimal required versions.

See https://packaging.python.org/discussions/install-requires-vs-requirements/

It is not considered best practice to use install_requires to pin dependencies to specific versions, or to specify sub-dependencies (i.e. dependencies of your dependencies). This is overly-restrictive, and prevents the user from gaining the benefit of dependency upgrades.