Uhm, now I see.. Tkinter already formats it correctly, and you shouldn't
be using itemconfigure for this task. If you try it directly in tk, like
this:

canvas .c
.c create text 0 0 -text {a b}
.c itemconfigure 1 -text

You would get something like this:

-text {} {} {} {a b}

While

.c itemcget 1 -text

Will return the same as Python: "a b"

Now what remains is to see how useful is to use itemconfigure for this,
and if it is worth making canvas.itemconfigure(id)['text'][-1] return "a
b" instead of ("a", "b"). Changing Misc._configure is too risky given
there are no tests for Tkinter (and I find it weird sometimes, someone
will still have to explain me why Tkinter plays with cnf and kw all the
time), the other option involves leaving these special needings in Text
and is something I dislike because other widgets could use these new
things that would be added. 
Yet another option would be to start writing unit tests for Tkinter and
much of these bugs would end up being caught and hopefully fixed properly.