Impossible to pass non-UTF-8 strings from vim to Python 3

When a vim variable has a value that is not a valid UTF-8 string, :py3 vim.eval('variable') raises UnicodeDecodeError.

This is causing plugins such as UltiSnips to crash when they try to process a list of mappings that involve <A-@>, since Vim thinks <A-@> is byte 0xC0. Here's what happens in the plugin:

• it runs vim.command('redir => _tmp_smaps | smap | redir END') to get the mappings
• it tries to parse vim.eval('_tmp_smaps').splitlines() to see what select-mode mappings there are

Now I've the following mapping in my .vimrc

map! <Esc>@ <A-@>

(I've many more, but this one seems to be the one causing the problem)

When I run smap, vim shows this mapping as

but when I do

:redir => tmp | smap <Esc>@ | redir END
:echo tmp

I see

and I can reproduce the crash with

:py3 import vim; vim.eval('tmp')

Traceback (most recent call last):
  File "<string>", line 1, in <module>
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xc0 in position 18: invalid start byte