make concurrent.ProcessPoolExecutor the default again

because loky gives issues on MacOS

Author: basnijholt

adaptive/runner.py

@@ -46,9 +46,7 @@
     asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
 
 
-_default_executor = (
-    loky.get_reusable_executor if with_loky else concurrent.ProcessPoolExecutor
-)
+_default_executor = concurrent.ProcessPoolExecutor
 
 
 class BaseRunner(metaclass=abc.ABCMeta):
@@ -62,7 +60,8 @@ class BaseRunner(metaclass=abc.ABCMeta):
         the learner as its sole argument, and return True when we should
         stop requesting more points.
     executor : `concurrent.futures.Executor`, `distributed.Client`,\
-               `mpi4py.futures.MPIPoolExecutor`, or `ipyparallel.Client`, optional
+               `mpi4py.futures.MPIPoolExecutor`, `ipyparallel.Client` or\
+               `loky.get_reusable_executor`, optional
         The executor in which to evaluate the function to be learned.
         If not provided, a new `~concurrent.futures.ProcessPoolExecutor`.
     ntasks : int, optional