Allow Python backgrounds threads to run in parallel with R

[t-kalinowski]

This PR allows spawned Python background threads to run while the main thread is executing R code.

For example, if you start a Python background thread like this:

library(reticulate)

py_config()

py_run_string(r"---(
import _thread
from time import sleep, localtime, strftime

def be_chatty():
  for i in range(100000):
    print('Hi', strftime('%H:%M:%S', localtime()))
    sleep(1)

_thread.start_new_thread(be_chatty, ())

)---")

With the current reticulate release, we don't see any output from the Python thread while at the R repl. The Python thread only is active while we are in a Python function that gives an opportunity for the GIL to be released (e.g, an explicit sleep, or evaluating Python bytecode). E.g., we only see output from the above background thread if we call in R:

py_sleep <- reticulate::import("time")$sleep
py_sleep(5)

However, with this PR, we now see output from the Python background thread even when not calling into Python from the main thread.

The primary motivation is to allow a Positron Python console to be backed by the same Python session reticulate in the Positron R console is using, without needing to block the R console, or spin-up+shutdown the ipykernel event loop on each call to repl_python().

[kevinushey]

LGTM!