Fix syntax of string format repr converters.

There's an error while `repr`-ing String or Comment nodes:

```
  File "tinycss2/ast.py", line 61, in __repr__
    return self.repr_format.format(self=self)
ValueError: Unknown format code 'r' for object of type 'unicode'
```

According to the [format string syntax](https://docs.python.org/3/library/string.html#format-string-syntax) "r" is a converter not a format spec.