Fix spurious blank GraceDb logging entries in Python 3

In Python 3, we started emitting a blank GraceDb message after every logging entry. For example, see https://gracedb-playground.ligo.org/events/view/T0191.

This is because of a change in logging.StreamHandler that caused a terminating newline to be written after every logging message, in a separate call to write(). See https://github.com/python/cpython/commit/2a20dfc2aabc3259d5b4276eeec91f83230fdcac.