Use raw-strings for regex strings to suppress python 3.12 warnings

[oboehmer]

python 3.12 complains about invalid escape sequences in regex strings, which should be specified as raw strings

.venv/lib/python3.12/site-packages/jsonpath_rw_ext/_string.py:19: SyntaxWarning: invalid escape sequence '\('
   SUB = re.compile("sub\(/(.*)/,\s+(.*)\)")
.venv/lib/python3.12/site-packages/jsonpath_rw_ext/_string.py:20: SyntaxWarning: invalid escape sequence '\('
   SPLIT = re.compile("split\((.),\s+(\d+),\s+(\d+|-1)\)")