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 Job ID Name Coverage
  Build
passed #556152
build:debian:jessie

00:03:44

passed #556153
build:debian:stretch

00:01:40

passed #556151
build:el7

00:01:23

passed #556150
build:tarball

00:00:39

 
  Test
passed #556158
docs

00:00:50

passed #556159
syntax

00:00:44

passed #556154
test:python2.7

00:00:41

passed #556155
test:python3.5

00:00:39

passed #556156
test:python3.6

00:00:40

passed #556157
test:python3.7

00:00:38