belopolsky will be happy to see this new version of my patch:
 - datetime.totimestamp() => (seconds, microseconds): two integers
 - datetime.totimestamp() implement don't use Python time.mktime() but 
directly the C version of mktime() because time.mktime() creates a 
float value
 - fix time.mktime() to support the timestamp -1 (first second before 
the epoch) to make it consistent with datetime.totimestamp() which 
also support this value
 - fix documentation: it's microseconds (10^-6) and not milliseconds 
(10^-3)