Fix multiprocessing support for Windows binary (#632)

* Fix multiprocessing support for Windows binary

The black and blackd binaries generated for Windows builds would fail on
reformatting multiple files due to a Windows-specific
multiprocessing issue. Fix by calling freeze_support() as
described in Python docs.

black.py

@@ -8,7 +8,7 @@
 import itertools
 import keyword
 import logging
-from multiprocessing import Manager
+from multiprocessing import Manager, freeze_support
 import os
 from pathlib import Path
 import pickle
@@ -3658,6 +3658,7 @@ def patch_click() -> None:
 
 
 def patched_main() -> None:
+    freeze_support()
     patch_click()
     main()
 

blackd.py

@@ -2,6 +2,7 @@
 from concurrent.futures import Executor, ProcessPoolExecutor
 from functools import partial
 import logging
+from multiprocessing import freeze_support
 
 from aiohttp import web
 import aiohttp_cors
@@ -130,6 +131,7 @@ async def handle(request: web.Request, executor: Executor) -> web.Response:
 
 
 def patched_main() -> None:
+    freeze_support()
     black.patch_click()
     main()