[BUG] Eastern hemisphere timezones break `std::chrono::time_point<std::chrono::system_clock, Duration>` caster

[YannickJadoul]

test_chrono_system_clock_roundtrip_time fails on my system between midnight and 1 o'clock.
Demonstrated by @henryiii in these tests (when run from a timezone in the eastern hemisphere): master...henryiii:fix/timebug

Further investigation shows that a negative number of microseconds is passed to PyDateTime_FromDateAndTime (in cast), resulting in an C API error that's not handled.

Current hypothesis of what goes wrong:

• Converting a Python datetime.time to C++ adds 1 January, 1970 as date in a std::tm struct.
• This is then converted to a <chrono> type through system_clock::from_time_t(std::mktime(...)), turning the clock back an hour to convert to UTC (I'm the CET time zone UTC+01:00 - ignoring summer time) .
• This results in a "negative" time. When converting back to Python, (duration_cast<us_t>(src.time_since_epoch() % seconds(1))).count() results in a negative number.