Performance issues from slow assertion

[andrew-vant]

A few days ago I had an issue where anytree operations significantly slowed down over time. cProfile attributed the added time to this assertion. If I'm reading it correctly, it makes adding a child node take O(N) time, where N is the number of its siblings-to-be.

That N gets rather large in my case. Probably I should refactor my code to avoid that, but I felt I should report it anyway. Setting PYTHONOPTIMIZE=TRUE (to disable assertions) does work as a workaround.