Remove the dependency on six module.

Since Botocore is now used to load data for Boto 3 there is no more need
in the existing codebase for the `six` module. This commit removes it and
removes the `boto3.compat` module. We can re-add it in the future if it
is needed.

This would be pulled instead of #41.

cc @jamesls, @kyleknap, @bosr