vulnfeeds: detect when version range has introduced > fixed

For PyPI vulnfeeds, there are some cases of bad data in the form of:

```
introduced: 1.0
fixed: 1.0b4
```

Encoded like so, this means that everything after and including 1.0 is affeted (because 1.0b4 comes *before* 1.0). This should instead be something like

```
introduced: 1.0a0
fixed: 1.0b4
```

We need to detect these cases in the vulnfeeds tool.