Python library: should depend on setuptools or otherwise replace `pkg_resources`

The python `asts` package depends internally on `pkg_resources` ([which is deprecated](https://setuptools.pypa.io/en/latest/pkg_resources.html)).

If still used, at least the `setuptools` package should be declared as a dependency. Otherwise, trying to pip-install & `import asts`, results in:

```shell
ModuleNotFoundError: No module named 'pkg_resources'
```