py-django-treebeard: updated to 4.4

Release 4.4

Implement a non-destructive path-fixing algorithm for MP_Node.fix_tree.
Ensure post_save is triggered after the parent node is updated in MP_AddChildHandler.
Fix static URL generation to use static template tag instead of constructing the URL manually.
Declare support for Django 2.2, 3.0 and 3.1.
Drop support for Django 2.1 and lower.
Drop support for Python 2.7 and Python 3.5.
Increase performance for MoveNodeForm when using large trees.

(adam)