msgpack.vim: require python3 on Windows

timestamp.strftime('%s') workaround only works on unix.

ref: https://github.com/neovim/neovim/pull/8371#discussion_r186311766

runtime/autoload/msgpack.vim

@@ -40,7 +40,7 @@ function s:msgpack_init_python() abort
     return s:msgpack_python_type
   endif
   let s:msgpack_python_initialized = 1
-  for suf in ['', '3']
+  for suf in (has('win32') ? ['3'] : ['', '3'])
     try
       execute 'python' . suf
               \. "\n"