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
```