bug#25753: 25.2; Python mode shell interaction not working 100%

[Carlos Pita]

Some years ago I proposed starting a thread python-side that provided
the completions through some kind of IPC, so as to not interfere with
prompt numbering and also to avoid blocking behavior (for example, if
eldoc tries to show documentation while python is running, it will
block or at least it would at the time). I started an implementation
then, sent a prototype to Fabian but it didn't get traction. I
reiterate my proposition now, even though I believe that's what
jupyter does nowadays in a much better way than any tiny
implementation of mine hacked into defcustoms will ever do.