- add PoolSingleton to __init__

- update readme

parfor/__init__.py

@@ -19,23 +19,18 @@ Result = TypeVar('Result')
 Iteration = TypeVar('Iteration')
 
 
+def select():
+    return nogil if hasattr(sys, '_is_gil_enabled') and not sys._is_gil_enabled() else gil  # noqa
+
+
 class ParPool:
     def __new__(cls, *args, **kwargs):
-        try:
-            if not sys._is_gil_enabled():  # noqa
-                return nogil.ParPool(*args, **kwargs)
-        except AttributeError:
-            pass
-        return gil.ParPool(*args, **kwargs)
+        return select().ParPool(*args, **kwargs)
 
 
-def nested():
-    try:
-        if not sys._is_gil_enabled():  # noqa
-            return nogil.Worker.nested
-    except AttributeError:
-        pass
-    return gil.Worker.nested
+class PoolSingleton:
+    def __new__(cls, *args, **kwargs):
+        return select().PoolSingleton(*args, **kwargs)
 
 
 class Chunks(Iterable):
@@ -222,7 +217,7 @@ def gmap(fun: Callable[[Iteration, Any, ...], Result], iterable: Iterable[Iterat
         bar_kwargs['desc'] = desc
     if 'disable' not in bar_kwargs:
         bar_kwargs['disable'] = not bar
-    if serial is True or (serial is None and len(iterable) < min(cpu_count, 4)) or nested():  # serial case
+    if serial is True or (serial is None and len(iterable) < min(cpu_count, 4)) or select().Worker.nested:  # serial case
 
         def tqdm_chunks(chunks: Chunks, *args, **kwargs) -> Iterable[Iteration]:  # noqa
             with tqdm(*args, **kwargs) as b: