bpo-39439: Fix multiprocessing spawn path in a venv on Windows (GH-18158)

diff --git a/Lib/multiprocessing/spawn.py b/Lib/multiprocessing/spawn.py
index 075f345..7cc129e 100644
--- a/Lib/multiprocessing/spawn.py
+++ b/Lib/multiprocessing/spawn.py
@@ -36,7 +36,7 @@
 if WINSERVICE:
     _python_exe = os.path.join(sys.exec_prefix, 'python.exe')
 else:
-    _python_exe = sys._base_executable
+    _python_exe = sys.executable
 
 def set_executable(exe):
     global _python_exe