EntryPoint.extras returns match objects and not the extras

```
>>> from importlib_metadata import EntryPoint
>>> ep = EntryPoint(name='foo', group='bar', value='package.module:attr [extra1, extra2]')
>>> ep.extras
[<re.Match object; span=(1, 7), match='extra1'>,
 <re.Match object; span=(9, 15), match='extra2'>]
```

extras should be a list of strings.