warning due to change in python3 syntax

Starting from, I believe, python 3.12, pmdtools throws an annoying warning:

```
SyntaxWarning: invalid escape sequence '\d' 
MDlist=re.findall('(\d+|\D+)',MD)
```

This should be easily fixed by just adding r (raw) like this:

```
MDlist=re.findall(r'(\d+|\D+)',MD)
```