I noticed that match.groupdict() returns string keys, even for a bytes regex:

```
>>> import re
>>> re.match(b"(?P<a>)", b"").groupdict()
{'a': b''}
```

This seems somewhat strange, because string and bytes matching in re are kind of two separate parts, cf. doc:

> Both patterns and strings to be searched can be Unicode strings (str) as well as 8-bit strings (bytes). However, Unicode strings and 8-bit strings cannot be mixed: that is, you cannot match a Unicode string with a byte pattern or vice-versa; similarly, when asking for a substitution, the replacement string must be of the same type as both the pattern and the search string.