specify urllib import in update_commdat.py

[scottstanie]

Only import urllib may not work in later versions of Python 3: https://stackoverflow.com/a/41217363/4174466

first run attempt produced:

$ python update_commdat.py
--------------------------------------
-->> Updating time conversion database 'commdat/[raw]_Leap_Second_History.dat':
Traceback (most recent call last):
  File "update_commdat.py", line 339, in <module>
    pt.update_etddt()
  File "update_commdat.py", line 270, in update_etddt
    urllib.request.urlopen(self.leapsec_rfile)
AttributeError: module 'urllib' has no attribute 'request'