SyntaxWarning with Python 3.12 and newer

[rhertzog]

With Python 3.12 and newer you get SyntaxWarning warnings when byte-compiling the hamster's code:

  /usr/lib/python3/dist-packages/hamster/lib/datetime.py:471:
SyntaxWarning: invalid escape sequence '\s'
    position="exact", separator="\s+", default_day=None, ref="now"):
  /usr/lib/python3/dist-packages/hamster/lib/datetime.py:472:
SyntaxWarning: invalid escape sequence '\s'
    """Parse a start-end range from text.
  /usr/lib/python3/dist-packages/hamster/widgets/activityentry.py:381:
SyntaxWarning: invalid escape sequence '\s'
    fragments = [f for f in re.split("[\s|#]", text)]
  /usr/lib/python3/dist-packages/hamster/widgets/timeinput.py:141:
SyntaxWarning: invalid escape sequence '\D'
    numbers = re.split("\D", str_time)

All those strings should be turned into regexp-quoted strings (with a leading r): r"\s+" for example.

(Initially reported in Debian here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1085633)