Replace \0 by \g<0> in python sub

> The back reference `\g<0>` substitutes in the entire substring matched by the RE.
http://docs.python.org/2/library/re.html#re.sub

The documentation does not mention` \0` as an alias to it (although `\1` to `\9` works). I guess this alias (`\0` <=> `\g<0>`) existed in old python version but it is not the case any more.

In python 2.7 and 3.3:
```python
> import re; re.compile(r'123').sub("@\0@","ab123cd")
'ab@\x00@cd' # KO
> import re; re.compile(r'123').sub("@\g<0>@","ab123cd")
'ab@123@cd' # OK
```