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

Running the 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;

contrib/signet/miner

@@ -30,7 +30,7 @@ logging.basicConfig(
 
 SIGNET_HEADER = b"\xec\xc7\xda\xa2"
 PSBT_SIGNET_BLOCK = b"\xfc\x06signetb"    # proprietary PSBT global field holding the block being signed
-RE_MULTIMINER = re.compile("^(\d+)(-(\d+))?/(\d+)$")
+RE_MULTIMINER = re.compile(r"^(\d+)(-(\d+))?/(\d+)$")
 
 def create_coinbase(height, value, spk):
     cb = CTransaction()