wwvbgen updates

I occasionally maintain a Python module called wwvbpy, which can generate WWVB timecodes for any desired time.

The change to winter (standard) time in the US is the perfect moment to review what's up with wwvpy and correct any discovered problems. (in fact, I was prompted by Chris's remark that his WWVB receiver had too eagerly applied the end of DST—as soon as GMT midnight rolled around, I think. Oops!)

This time around, here are the minor tweaks I made:

Update license to GPL3+ (was GPL2+)
Revamp how DUT1 ("iersdata") is gathered and harmonized
Convert tests to run as unittests; fix timezone test problems
Add github actions for CI
Add github "cron" to update iersdata
Add "expected output" tests for phase data, though this is just self-generated, not independently-generated data

WWVB timecode: year=20 days=306 hour=06 min=50 dst=1 ut1=-200 ly=1 ls=0 --style=duration
'20+306 06:50  852522222822222255282255222228255222252822522225282222252258