Running Xcode memory graphs and the Instruments tools revealed
memory leaks caused, in the main, by over-retained objects.
This removes the unnecessary 'retains' and adds some asserts
to guard against over-retaining in the future.
There are a few memory leaks remaining involving PyUnicode_DecodeUTF8
but I am unable to identify the cause of these at this time.
Authored by Apple: James McCarthy
Pull Request: https://projects.blender.org/blender/blender/pulls/129117
7fbc9e9428