gh-110850: Cleanup PyTime API: PyTime_t are nanoseconds (#115753)

PyTime_t no longer uses an arbitrary unit, it's always a number of
nanoseconds (64-bit signed integer).

* Rename _PyTime_FromNanosecondsObject() to _PyTime_FromLong().
* Rename _PyTime_AsNanosecondsObject() to _PyTime_AsLong().
* Remove pytime_from_nanoseconds().
* Remove pytime_as_nanoseconds().
* Remove _PyTime_FromNanoseconds().