[Windows] PySide2 not working with Python-3.8+

Platform: `Windows 10`
Rez version: `2.103.4`

**This issue is not Rez-dependent but this is strongly related to its way of working (separated packages).**

## TL;DR

As far as I know, the use of `PySide2` with `Python-3.8+` via `Rez` seems impossible on Windows.

## Description

I recently installed `Rez` on my Windows 10 computer. To follow the [VFX Platform](https://vfxplatform.com/) recommendations, I wanted to work with `Python-3.9` and `PySide2-5.15`.

After installing `PySide2` (and its dependency `shiboken2`) via `rez-pip`, I tried to run a Python shell and to `import PySide2`.
I instantly got an exception coming from the `PySide2\__init__.py` file. Actually, as we can see in the code [right here](https://code.qt.io/cgit/pyside/pyside-setup.git/tree/sources/pyside2/PySide2/__init__.py.in?h=5.15.2#n53), when using `Python-3.8+`, the file tries to deal with relative paths from `PySide2` to find the `shiboken` directory and, because it does not find it, it raises [this exception](https://code.qt.io/cgit/pyside/pyside-setup.git/tree/sources/pyside2/PySide2/__init__.py.in?h=5.15.2#n26).

I am not sure this is the good place to talk about this but I thought it was quite relevant to notice.

Reproduce on Windows 10 with Rez 2.103.4, Python 3.9, and PySide2 5.15 plus shiboken2 installed through rez-pip, then run a Python shell and import PySide2. Inspect PySide2\__init__.py and the linked upstream PySide2 setup file to compare the failing path behavior. Done should establish whether Rez is responsible and document a concrete resolution or limitation.

Written by the indexing model from the issue text.