Revert "Introduce six to address python 3 compatibility."

This reverts commit e06e1719cfeb15a9c6425abcf567acb0ffda032f.

Conflicts:
	crispy_forms/templatetags/crispy_forms_utils.py
`force_unicode` is now `force_text`.
Minor python3 compatibility fixes.
Use compatibility layer for python 3.
Fix ContainerHolder.first_container_with_errors in python 3.

This is a bit of an odd one. To be honest, I'm not sure why
it didn't work in python 3, but I think it's prettier this
way anyway.