* 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