This change replaces the deprecated `pkg_resources` (part of
`setuptools`) with `importlib.metadata` for Python 3.8 and above.

Switching to `importlib.metadata` allows us to drop the test-dependency
on `setuptools` and gets rid of the `DeprecationWarning` raised by
certain versions of `pkg_resources` when being imported:

```
DeprecationWarning: pkg_resources is deprecated as an API
```