valgrind-monitor.py regular expressions should use raw strings

valgrind-monitor.py regular expressions should use raw strings

With python 3.12 gdb will produce the following SyntaxWarning when
loading valgrind-monitor-def.py:

  /usr/share/gdb/auto-load/valgrind-monitor-def.py:214:
  SyntaxWarning: invalid escape sequence '\['
    if re.fullmatch("^0x[0123456789ABCDEFabcdef]+\[[^\[\]]+\]$", arg_str):

In a future python version this will become an SyntaxError.

Use a raw strings for the regular expression.

https://bugs.kde.org/show_bug.cgi?id=476708