SyntaxWarning on Python 3.12

[sergei-maertens]

I'm checking if I can upgrade a project to Python 3.12 yet, and while doing so, this library shows some syntax warnings:

2.508 /usr/local/lib/python3.12/site-packages/precise_bbcode/bbcode/placeholder.py:37: SyntaxWarning: invalid escape sequence '\w'                                                            
2.508   if not re.match('^[\w]+$', new_placeholder.name):                                                                                                                                     
2.510 /usr/local/lib/python3.12/site-packages/precise_bbcode/bbcode/tag.py:46: SyntaxWarning: invalid escape sequence '\s'                                                                    
2.510   if not re.match('^[^\s=]+$', new_tag.name):   

I believe these should be updated to raw strings, so adding a r -prefix should be sufficient.