Versioning extension for SQLAlchemy.
It seems that older versions of SQLAlchemy and SQLAlchemy-Utils depended on six, so there was no need to declare it here. However, latest releases of SQLAlchemy and SQLAlchemy-Utils no longer depend on six, but SQLAlchemy-Continuum does in fact [import six](https://github.com/kvesteri/sqlalchemy-continuum/blob/master/sqlalchemy_continuum/operation.py#L7), so we need to declare it as a dependency.
This issue appears to be discussing a feature request or bug report related to the repository. Based on the content, it seems to be resolved. The issue was opened by zupo and has received 1 comments.