Use `pyproject.toml` for PEP 517 / PEP 518 metadata

Currently this project [uses a common `setup.py`](https://github.com/AnswerDotAI/nbdev-index/blob/master/setup.py) for each of its packages.

Please consider migrating to the standardardised `pyproject.toml`, as installing by running `./setup.py` has been deprecated for several years.

The minimum required is generating a `pyproject.toml` file that contains the below:

```toml
[build-system]
requires = ["setuptools>=42"]
build-backend = "setuptools.build_meta"
```

This is all that's required to move to standardised build metadata, away from the legacy `setup.py` approach.

----

You could also consider using standard project metadata, via the `[metadata]` table, and a different build backend, such as `flit-core`, which is smaller and a little faster. See e.g. https://flit.pypa.io/en/stable/pyproject_toml.html#new-style-metadata.

A

Inspect the existing setup.py and package layout to determine where the shared packaging configuration applies. Add the requested pyproject.toml build-system metadata, then verify that package installation works without invoking setup.py.

Written by the indexing model from the issue text.