www/py-dj42-django-treebeard: Add py-dj42-django-treebeard 4.7.1 (copied from py-django-treebeard)

django-treebeard is a library that implements efficient tree implementations for
the Django Web Framework 1.8 and later.

Features:

- Flexible: Includes 3 different tree implementations with the same API:
  * Adjacency List
  * Materialized Path
  * Nested Sets
- Fast: Optimized non-naive tree operations
- Easy: Uses Django Model Inheritance with abstract classes to define your own
	models.
- Clean: Testable and well tested code base. Code/branch test coverage is
	 above 96%.