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().
The following steps have been taken:
1. Black was updated to latest version. The code has been formatted with
the new version.
2. The pyupgrade utility is installed. This helped to remove all the
code that was needed to support Python < 3.7.
Fix#541.
Co-authored-by: Inada Naoki <songofacandy@gmail.com>
