In fact, the fix just switches off C Accelerators for the library allowing PyInstaller to import the Pure Python version.
It works but the performance slowdown is an order of tens.

Digging into the problem more, I've figured out that PyInstaller has hooks for a bunch of popular libraries to make them work.
These hooks do a dirty monkey patching. Sure, "multidict" is not covered by builtin hooks and had a problem.
As a "multidict" maintainer I'm not interested in supporting the required patch in PyInstaller, switching to a slow-but-working version is totally fine with me.
The story for "multidict" is done.

With the CPython developer hat on, I very doubt if PyInstaller can be included in the standard library.
The project exists and it is useful for many people, this is great.
But it requires maintaining very many hooks for every non-trivial library to keep the PyInstaller working.
This means that we, CPython core developers, should carry this maintenance burden if PyInstaller becomes a part of CPython.
We will need to adopt and patch an endless list of libraries that are fine and works perfectly themself but have problems when packed with PyInstaller.

Sorry, I think it never happened.

Say again, PyInstaller exists. It is available on PyPI and can be installed easily by pip. 
PyInstaller does amazing work.
But by the PyInstaller nature, it requires the maintenance of very many hooks for every non-trivial Python library.
This raises an extremely high bar for the inclusion of the library into the standard batteries.
Better to keep it as-is, as a third-party tool installable from PyPI.