use python.exe, not python, when accessing the interpreter in a venv under windows

docs/news.txt

@@ -36,6 +36,8 @@ hg tip
   repositories and ability to freeze them. Refactored support for version
   control systems.
 
+* Fixed Win32 virtualenv (``-E``) option.
+
 0.2.1
 -----
 

pip.py

@@ -936,7 +936,7 @@ def restart_in_venv(venv, args):
         ## FIXME: always have no_site_packages?
         virtualenv.create_environment(venv, site_packages=False)
     if sys.platform == 'win32':
-        python = os.path.join(venv, 'Scripts', 'python')
+        python = os.path.join(venv, 'Scripts', 'python.exe')
     else:
         python = os.path.join(venv, 'bin', 'python')
     if not os.path.exists(python):