Delete a warning with Python 3.12

[benoit9126]

With Python 3.12, a warning is emitted from the settings.py file.

grappelli/settings.py:27: SyntaxWarning: invalid escape sequence '\d'
  SWITCH_USER_REGEX = getattr(settings, "GRAPPELLI_SWITCH_USER_REGEX", "\d+")

A simple raw string solves it.