fix: reject stray pipe in sign by using [+-] character class

Inside a character class '|' is a literal, so [+|-] matched '+', '-', or
'|'. A leading pipe was captured as a sign and stripped, letting malformed
input like '|5m' parse to 300 instead of returning None. Restrict the
class to the two intended sign characters.

pytimeparse/timeparse.py

@@ -33,7 +33,7 @@ kinds of time expressions.
 
 import re
 
-SIGN        = r'(?P<sign>[+|-])?'
+SIGN        = r'(?P<sign>[+-])?'
 #YEARS      = r'(?P<years>\d+)\s*(?:ys?|yrs?.?|years?)'
 #MONTHS     = r'(?P<months>\d+)\s*(?:mos?.?|mths?.?|months?)'
 WEEKS       = r'(?P<weeks>[\d.]+)\s*(?:w|wks?|weeks?)'