SyntaxWarning: invalid escape sequence '\d'

Python warning interprets \d as an invalid escape sequence.

Ex:
``` 
/home/pins-dev/Projects/lightning-payencode/lnaddr.py:54: SyntaxWarning: invalid escape sequence '\d'
  if not re.fullmatch("\d+[pnum]?", str(amount)):
/home/pins-dev/Projects/lightning-payencode/lnaddr.py:273: SyntaxWarning: invalid escape sequence '\d'
  m = re.search("[^\d]+", hrp[2:])
```