[$] Deep immutability for Python

Python has recently seen a number of experiments to improve its parallelperformance, including exposingsubinterpreters as part of the standard library. These allowseparate threads within the same Python process to run simultaneously, as longas any data sent between them is copied, rather than shared.PEP795 ("Deep Immutability in Python")seeks to make efficient sharing of data between subinterpreters possible byallowing Python objects to be "frozen", so that they can be accessed frommultiple subinterpreters without copying or synchronization.That task is more difficult than itseems, and the PEP prompted a good deal of skepticism from the Python community.