`Requirement('installer')` errors after 2bd5da391c302f2f5a18fd3e2bd9fb3b75c02e34

```python
from packaging.requirements import Requirement
Requirement('installer')
```

The regression has been introduced in [2bd5da391c302f2f5a18fd3e2bd9fb3b75c02e34](https://github.com/pypa/packaging/commit/2e5593c1288b9f1b57bb1f79acc0ed5fd1b7009c).