Using recently added pytest.PY2 instead of str is bytes.

[rwgk]

Important gain: uniformity & therefore easier cleanup when we drop PY2 support.
Very slight loss: it was nice to have str is bytes as a reminder in this specific context.

[EricCousineau-TRI]

Looks great!

[EricCousineau-TRI]

Failure here is for Python 3.9-dev on macOS:
https://github.com/pybind/pybind11/pull/2396/checks?check_run_id=986425511#step:14:56

     def test_python_to_cpp_to_python_from_thread_multiple_parallel():
        """Makes sure there is no GIL deadlock when running in a thread multiple times in parallel.
    
        It runs in a separate process to be able to stop and assert if it deadlocks.
        """
>       assert _run_in_process(_python_to_cpp_to_python_from_threads, 8, parallel=True) == 0
E       assert -11 == 0
E        +  where -11 = _run_in_process(_python_to_cpp_to_python_from_threads, 8, parallel=True)

test_gil_scoped.py:65: AssertionError

I say this is good to merge, as the failure is unrelated to this PR?

[EricCousineau-TRI]

I'ma go ahead and pull the "merge" trigger.

[rwgk]

Awesome, thanks Eric!