[$] Deep immutability for Python

Python has recently seen a number of experiments to improve its parallel
performance, including exposing

subinterpreters as part of the standard library. These allow
separate threads within the same Python process to run simultaneously, as long
as any data sent between them is copied, rather than shared.

PEP 795 ('Deep Immutability in Python')
seeks to make efficient sharing of data between subinterpreters possible by
allowing Python objects to be 'frozen', so that they can be accessed from
multiple subinterpreters without copying or synchronization.
That task is more difficult than it
seems, and the PEP prompted a good deal of skepticism from the Python community.