use a concurrent.ProcessPoolExecutor on Windows too

In 890f84c I wrote that a 'concurrent.futures.ProcessPoolExecutor' doesn't work on Windows.
However, it seems like it just works now.

adaptive/runner.py

@@ -2,7 +2,6 @@
 import asyncio
 import concurrent.futures as concurrent
 import inspect
-import os
 import pickle
 import sys
 import time
@@ -39,30 +38,6 @@
     asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
 
 
-if os.name == "nt":
-    if with_distributed:
-        _default_executor = distributed.Client
-        _default_executor_kwargs = {"address": distributed.LocalCluster()}
-    else:
-        _windows_executor_msg = (
-            "The default executor on Windows for 'adaptive.Runner' cannot "
-            "be used because the package 'distributed' is not installed. "
-            "Either install 'distributed' or explicitly specify an executor "
-            "when using 'adaptive.Runner'."
-        )
-
-        _default_executor_kwargs = {}
-
-        def _default_executor(*args, **kwargs):
-            raise RuntimeError(_windows_executor_msg)
-
-        warnings.warn(_windows_executor_msg)
-
-else:
-    _default_executor = concurrent.ProcessPoolExecutor
-    _default_executor_kwargs = {}
-
-
 class BaseRunner(metaclass=abc.ABCMeta):
     r"""Base class for runners that use `concurrent.futures.Executors`.
 
@@ -76,9 +51,7 @@ class BaseRunner(metaclass=abc.ABCMeta):
     executor : `concurrent.futures.Executor`, `distributed.Client`,\
                `mpi4py.futures.MPIPoolExecutor`, or `ipyparallel.Client`, optional
         The executor in which to evaluate the function to be learned.
-        If not provided, a new `~concurrent.futures.ProcessPoolExecutor`
-        is used on Unix systems while on Windows a `distributed.Client`
-        is used if `distributed` is installed.
+        If not provided, a new `~concurrent.futures.ProcessPoolExecutor`.
     ntasks : int, optional
         The number of concurrent function evaluations. Defaults to the number
         of cores available in `executor`.
@@ -298,9 +271,7 @@ class BlockingRunner(BaseRunner):
     executor : `concurrent.futures.Executor`, `distributed.Client`,\
                `mpi4py.futures.MPIPoolExecutor`, or `ipyparallel.Client`, optional
         The executor in which to evaluate the function to be learned.
-        If not provided, a new `~concurrent.futures.ProcessPoolExecutor`
-        is used on Unix systems while on Windows a `distributed.Client`
-        is used if `distributed` is installed.
+        If not provided, a new `~concurrent.futures.ProcessPoolExecutor`.
     ntasks : int, optional
         The number of concurrent function evaluations. Defaults to the number
         of cores available in `executor`.
@@ -417,9 +388,7 @@ class AsyncRunner(BaseRunner):
     executor : `concurrent.futures.Executor`, `distributed.Client`,\
                `mpi4py.futures.MPIPoolExecutor`, or `ipyparallel.Client`, optional
         The executor in which to evaluate the function to be learned.
-        If not provided, a new `~concurrent.futures.ProcessPoolExecutor`
-        is used on Unix systems while on Windows a `distributed.Client`
-        is used if `distributed` is installed.
+        If not provided, a new `~concurrent.futures.ProcessPoolExecutor`.
     ntasks : int, optional
         The number of concurrent function evaluations. Defaults to the number
         of cores available in `executor`.
@@ -773,7 +742,7 @@ def shutdown(self, wait=True):
 
 def _ensure_executor(executor):
     if executor is None:
-        executor = _default_executor(**_default_executor_kwargs)
+        executor = concurrent.ProcessPoolExecutor()
 
     if isinstance(executor, concurrent.Executor):
         return executor

setup.py

@@ -1,6 +1,5 @@
 #!/usr/bin/env python3
 
-import os
 import sys
 
 from setuptools import find_packages, setup
@@ -31,10 +30,6 @@ def get_version_and_cmdclass(package_name):
     "atomicwrites",
 ]
 
-if os.name == "nt":  # on Windows
-    # distributed provides the default executor on Windows
-    install_requires.append("distributed")
-
 extras_require = {
     "notebook": [
         "ipython",