Migrate features compliant with six concept to `xoutil.eight`¶

Todo

Check all around xoutil and migrate to xoutil.eight all tools that are related to write code that will be compatible with Python 2 and 3 versions.

This task was already started by migrating:

Meta-classes definition. See xoutil.eight.meta.metaclass().
Definitions that exists in only one version (2 or 3) of Python module types. See xoutil.eight.types.

Todo

Progressively update HISTORY file with all xoutil.eight related progress.

Migrate features compliant with six concept to xoutil.eight¶