This change adds build-system metadata to pyproject.toml and
ensures the dir for setup.py is in sys.path.

The sys.path modification is to ensure the project ./tools/
modules can be imported in setup.py during editable install.

With this tweak to the project metadata, it should be
possible to install pyqtgraph with an editable install,
using setuptools

```
mkdir newproject; cd newproject
python3 -m venv --prompt newproject env
source env/bin/activate
pip install -e ../pyqtgraph/ --config-settings editable_mode=compat
```

After sending a push request for the previous change in this
patch branch, I noticed the following in the build log for Read the Docs:

> PyQtGraph requires one of PyQt5, PyQt6, PySide2 or PySide6; none of these packages could be imported.

Of course PyQt6 is listed in doc/requirements.txt and it was installed under that step:

> python -m pip install --exists-action=w --no-cache-dir -r doc/requirements.txt
> [...]
> Successfully installed PyQt6-6.6.0 [...]

Maybe this change produced some kind of a side effect for how the rtd
build resolves the imports for the docs build?

So here is a first guess, to try to unbreak it if the change
would break the docs build. Appending to sys.path instead of
adding the project root path to the front of sys.path

With the dependency `PyQt6==6.6.0` pip may install a
PyQt6-Qt6 version greater than the PyQt6 version, e.g
6.6.1.

This version mismatch may result in an errors after
loading the PyQt6 libraries, such as seen when running
the pyqtgraph tests with Python 3.12

> E   Exception: PyQtGraph requires one of PyQt5, PyQt6, PySide2 or PySide6; none of these packages could be imported.

After adding a dependency line to ensure a version parity for
the pyqt6 and pyqt6-qt6 installations with pip, then installing
with `pip -r doc/requirements.txt` the pyqtgraph tests and the docs
build should run successfully with PyQt6 in Python 3.12

updating pyqt6, pyqt6-qt6 version deps for the test-pip gh
workflow and in tox.ini

This may serve to address the error seen under the rtd docs
build, such that might also show up in the test-pip build:

> Exception: PyQtGraph requires one of PyQt5, PyQt6, PySide2 or PySide6; none of these packages could be imported.

Ensuring an exact version match for pyqt6 and pyqt6-qt6
depdencies in test-pip and in tox.ini