[mattip]

PyPy is a different implementation of a python interpreter, it includes a JIT to rewrite hot code into assembler. See www.pypy.org for more info. The JIT cannot be tacked on to CPython (what you call python3) since it is part of the interpreter.

[cwillu]

The jit is _generated_ from the interpreter; you can actually run pypy's interpreter directly from cpython, it's just slower.