Fix strftime format string (#646)

* Fix strftime format string

Lines 28 and 29 that were using `.strftime('%s')` where it should be written as `.strftime('%S')`. This error would make twint fail with an `Invalid format string` when using time-related configurations like `Since` and `Until`.

* Change strftime to timestamp

As we are looking for the timestamp version of the provided date/time string, using `timestamp()` should be more straightforward and shouldn't create any incompatibility issues as twint already requires Python >= 3.6

twint/output.py

@@ -35,8 +35,8 @@ def datecheck(datetimestamp, config):
     logme.debug(__name__+':datecheck')
     if config.Since and config.Until:
         logme.debug(__name__+':datecheck:dateRangeTrue')
-        d = int(datetime.strptime(datetimestamp, "%Y-%m-%d %H:%M:%S").strftime('%s'))
-        s = int(datetime.strptime(config.Since, "%Y-%m-%d %H:%M:%S").strftime('%s'))
+        d = int(datetime.strptime(datetimestamp, "%Y-%m-%d %H:%M:%S").timestamp())
+        s = int(datetime.strptime(config.Since, "%Y-%m-%d %H:%M:%S").timestamp())
         if d < s:
            return False
     logme.debug(__name__+':datecheck:dateRangeFalse')