New src file name

The latest 0.8.1 release uses a new src file name:

```
python_picard-0.8.1.tar.gz
```

Previously, the underscore was a dash:

```
python-picard-0.8.tar.gz
```

Nothing serious, but it broke my AUR package build. I already adapted it, so no need to change anything, I just wanted to let you know in case this change was unintentional.