ligo-gracedb.spec: use python-srpm-macros

easy to resolve dependencies when building a src.rpm
11 jobs for add-missing-dependencies in 4 minutes and 37 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #395584
build:debian:jessie

00:03:35

passed #395585
build:debian:stretch

00:01:27

passed #395583
build:el7

00:01:09

passed #395582
build:tarball

00:00:26

 
  Test
passed #395591
docs

00:01:00

passed #395592
syntax

00:00:52

passed #395586
test:python2.7

00:00:39

passed #395587
test:python3.4

00:00:49

passed #395588
test:python3.5

00:00:51

passed #395589
test:python3.6

00:00:34

passed #395590
test:python3.7

00:00:43