For some reason sys.stdin may be None on Windows, and makes test_multiprocessing fail.
Since we are closing the fileno anyway, the best is to skip this part.

Now test_multiprocessing should pass on Windows.
diff --git a/Lib/multiprocessing/process.py b/Lib/multiprocessing/process.py
index e21d2f0..4a06a45 100644
--- a/Lib/multiprocessing/process.py
+++ b/Lib/multiprocessing/process.py
@@ -219,10 +219,11 @@
         try:
             self._children = set()
             self._counter = itertools.count(1)
-            try:
-                os.close(sys.stdin.fileno())
-            except (OSError, ValueError):
-                pass
+            if sys.stdin is not None:
+                try:
+                    os.close(sys.stdin.fileno())
+                except (OSError, ValueError):
+                    pass
             _current_process = self
             util._finalizer_registry.clear()
             util._run_after_forkers()