Fix #9513 to remove relative imports from multiprocessing.

The test suite currently skips test_multiprocessing on Windows
because it fails on finding _multiprocessing in several
win32-specific blocks. Removing the relative import lets this
through and allows the test to run (and pass).
diff --git a/Lib/multiprocessing/reduction.py b/Lib/multiprocessing/reduction.py
index 1813729..60e7151 100644
--- a/Lib/multiprocessing/reduction.py
+++ b/Lib/multiprocessing/reduction.py
@@ -34,7 +34,7 @@
 
 if sys.platform == 'win32':
     import _subprocess
-    from ._multiprocessing import win32
+    from _multiprocessing import win32
 
     def send_handle(conn, handle, destination_pid):
         process_handle = win32.OpenProcess(