Use python3 if available

This commit incorporates [fievel's work](0863e1434e) to use python3 if possible.

I have not personally test vim+python3 but overdue dates still works with
python2.

syntax/python/dateregex/dateregex/before.py

@@ -24,6 +24,7 @@ def _year_regex_before(year):
 
     year_regex += ')'
     return '-'.join((year_regex, r'\d{2}', r'\d{2}'))
+
 def _month_regex_before(year, month):
     if month == '01':
         return None
@@ -63,6 +64,7 @@ def _day_regex_before(year, month, day):
 
 
 
+
 def regex_date_before(given_date):
     year, month, day = given_date.isoformat().split('-')