Improve definitions for unicodedata

```console
rr-@tornado:~$ cat test.py
import unicodedata
print(unicodedata.category('A'))
rr-@tornado:~$ mypy test.py --disallow-untyped-calls
test.py:2: error: Call to untyped function "category" in typed context
```

I think this happens because https://github.com/python/typeshed/blob/master/stdlib/3/unicodedata.pyi is too lax about returned types.