Drop support for Python 3.4

* Python 3.4 reached end of life on March 18, 2019.
* The ligo-gracedb unit tests do not pass under Python 3.4.
10 jobs for drop-python-3.4 in 4 minutes and 37 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build:debian:jessie #556152

00:03:44

passed build:debian:stretch #556153

00:01:40

passed build:el7 #556151

00:01:23

passed build:tarball #556150

00:00:39

 
  Test
passed docs #556158

00:00:50

passed syntax #556159

00:00:44

passed test:python2.7 #556154

00:00:41

passed test:python3.5 #556155

00:00:39

passed test:python3.6 #556156

00:00:40

passed test:python3.7 #556157

00:00:38