Initial version, using the LP RTC clock. It provides setting the date and
time with rtc.init() or rtc.datetime(), and reading the date and time with
rtc.datetime() or rtc.now(). The method weekday() reports the weekday of
the current date. It starts with 0 for Monday.
The tuple order for datetime() and now() matches the CPython sequence:
(year, month, day, hour, minute, second, microsecond, TZ). TZ is ignored
and reported as None. Microsecond is provided at a best effort.
If a battery is not supplied, the default boot date/time is 1970/1/1 0:0:0.
With a battery, the clock continues to run even when the board is not
powered. The clock is quite precise. If not, using rtc.calibration() may
help.
