Parsing/formatting ISO 8601 years less than zero or greater than 9999

Not entirely sure if you should care about such years but I noticed while playing around that `{parttime}` doesn't always correctly parse/format ISO 8601 years less than zero or greater than 9999

[According to Wikipedia](https://en.wikipedia.org/wiki/ISO_8601#Years) years less than zero or greater than 9999 should be at least four digits wide and prependend with either a "+" or a "-" e.g.

```
+12345
-12345
-0004
```

I do observe that negative numbers strictly less than `-999` seem to be formatted correctly.