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%.