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/heap.py b/Lib/multiprocessing/heap.py
index ab6cf16..5d27b27 100644
--- a/Lib/multiprocessing/heap.py
+++ b/Lib/multiprocessing/heap.py
@@ -26,7 +26,7 @@
 
 if sys.platform == 'win32':
 
-    from ._multiprocessing import win32
+    from _multiprocessing import win32
 
     class Arena(object):