Fix Timestamp.from_datetime returning wrong value for pre-epoch times

int() truncates towards zero, so for pre-epoch datetimes with non-zero
microseconds the seconds component gets the wrong value. For example,
datetime(1969, 12, 31, 23, 59, 59, 500000, UTC) has timestamp -0.5,
but int(-0.5) == 0, producing Timestamp(0, 500000000) (+0.5s after
epoch) instead of Timestamp(-1, 500000000) (-0.5s before epoch).

Use floor division (// 1) instead, consistent with from_unix().

Author: bysiber

msgpack/ext.py

@@ -167,4 +167,4 @@ def from_datetime(dt):
 
         :rtype: Timestamp
         """
-        return Timestamp(seconds=int(dt.timestamp()), nanoseconds=dt.microsecond * 1000)
+        return Timestamp(seconds=int(dt.timestamp() // 1), nanoseconds=dt.microsecond * 1000)