contrib: use a raw string for a regular expression literal that contains backslashes in signet/miner

Running contrib/signet/miner under python >= 3.12 causes a SyntaxWarning. The problem was already present in previous versions, but it only triggered a DeprecationWarning, which was not shown by default.

The change is useful for future-proofing the code base, since future python versions will start to exit with a runtime exception (see the reference given later).

Command to see the warning at runtime under python3.11 (DeprecationWarning, needs "-Walways"):

$ python3.11 -Walways ./contrib/signet/miner
<BASE>/contrib/signet/miner:33: DeprecationWarning: invalid escape sequence '\d'
  RE_MULTIMINER = re.compile("^(\d+)(-(\d+))?/(\d+)$")
2023-11-15 16:02:49 ERROR Must specify command

Command to see the warning at runtime under python3.12 (SyntaxWarning, no modifiers needed):

$ python3.12 ./contrib/signet/miner
<BASE>/contrib/signet/miner:33: SyntaxWarning: invalid escape sequence '\d'
  RE_MULTIMINER = re.compile("^(\d+)(-(\d+))?/(\d+)$")
2023-11-15 16:03:00 ERROR Must specify command

Reference (https://docs.python.org/3.8/library/re.html):

Regular expressions use the backslash character ('') [...]. This collides with Python’s usage of the same character for the same purpose in string literals; [...]

Also, please note that any invalid escape sequences in Python’s usage of the backslash in string literals now generate a DeprecationWarning and in the future this will become a SyntaxError.

The solution is to use Python’s raw string notation for regular expression patterns;