BF: fix bug for Windows bat-file install

Windows .bat file shadow of Python script fails for paths with spaces.

Thanks to P. Renner for the fix - see
matthew-brett/myscripter#2

Author: matthew-brett

nisext/sexts.py

@@ -222,7 +222,7 @@ def _package_status(pkg_name, version, version_getter, checker):
 exit /b 1
 :goodstart
 set py_exe=%line1:~2%
-call %py_exe% %pyscript% %*
+call "%py_exe%" %pyscript% %*
 """
 
 class install_scripts_bat(install_scripts):