Add a Centos runner

Centos is apparently different enough from fedora when running tests.
14 jobs for richardmaw/centos-oldgit-test-fixes in 47 minutes and 34 seconds (queued for 7 minutes and 2 seconds)
Status Job ID Name Coverage
  Test
passed #147864399
code_quality

00:04:10

passed #147864398
docs

00:06:11

passed #147864397
lint

00:01:54

passed #147864389
tests-centos-7.6

00:43:49

passed #147864380
tests-debian-9

00:25:50

passed #147864382
tests-fedora-27

00:26:33

passed #147864384
tests-fedora-28

00:27:09

passed #147864393
tests-fedora-missing-deps

00:12:48

passed #147864395
tests-fedora-update-deps

00:27:56

passed #147864388
tests-python-3.7-stretch

00:26:02

passed #147864386
tests-ubuntu-18.04

00:25:11

passed #147864391
tests-unix

00:21:01

 
  Post
passed #147864400
analysis

00:01:00

passed #147864401
coverage

00:00:44

86.04%