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/forking.py b/Lib/multiprocessing/forking.py
index 5d66fa3..a7e8eb7 100644
--- a/Lib/multiprocessing/forking.py
+++ b/Lib/multiprocessing/forking.py
@@ -157,7 +157,7 @@
     import time
 
     from pickle import dump, load, HIGHEST_PROTOCOL
-    from ._multiprocessing import win32, Connection, PipeConnection
+    from _multiprocessing import win32, Connection, PipeConnection
     from .util import Finalize
 
     def dump(obj, file, protocol=None):