Example no-site flag for subinterpreters module

[tonybaloney]

This is just example code, but with a flag for no-site propagating down to the interpreter config, sub interpreters are 50% faster to create because init_import_site() takes 50% of the execution time for _Py_InterpreterNewFromConfig

For cases where users want a site in the main interpreter, but don't need a site in the sub interpreters, it would be very helpful to have a flag.

I've created a benchmark to test this against sub interpreters, threading and multiprocessing (skip those other 2 to just see a comparison to sub interpreters).

If you like this idea I'm happy to submit a proper PR with tests.

import pyperf
from multiprocessing import Process
from threading import Thread
import _xxsubinterpreters as subinterpreters

def f():
    ...


def bench_threading(n):
    # Code to launch specific model
    for _ in range(n):
        t = Thread(target=f)
        t.start()
        t.join()

def bench_subinterpreters(n, site=True):
    # Code to launch specific model
    for _ in range(n):
        sid = subinterpreters.create(site=site)
        subinterpreters.run_string(sid, "")

def bench_multiprocessing(n):
    # Code to launch specific model
    for _ in range(n):
        t = Process(target=f)
        t.start()
        t.join()

if __name__ == "__main__":
    runner = pyperf.Runner()
    runner.metadata['description'] = "Benchmark execution models"
    n = 100
    runner.bench_func('threading', bench_threading, n)
    runner.bench_func('subinterpreters', bench_subinterpreters, n)
    runner.bench_func('subinterpreters_nosite', bench_subinterpreters, n, False)
    runner.bench_func('multiprocessing', bench_multiprocessing, n)