ci: reduce duplications in .gitlab-ci.yml

by using the YAML special anchors feature, duplication in .gitlab-ci.yml
could greatly be reduced
13 jobs for 44-reduce-complexity-of-gitlab-ci-yml in 20 minutes and 52 seconds (queued for 5 seconds)
Status Name Job ID Coverage
  Test
passed py3.5-min-mysql #19764

00:10:44

92.0%
passed py3.5-min-postgresql #19762

00:10:34

92.0%
passed py3.5-min-sqlite #19760

00:10:27

91.0%
passed py3.5-pypi-mysql #19765

00:10:54

92.0%
passed py3.5-pypi-postgresql #19772

00:09:56

92.0%
passed py3.5-pypi-sqlite #19761

00:10:42

91.0%
passed py3.6-min-mysql #19770

00:10:46

92.0%
passed py3.6-min-postgresql #19768

00:10:35

92.0%
passed py3.6-min-sqlite #19766

00:10:35

91.0%
passed py3.6-pypi-mysql #19771

00:10:50

92.0%
passed py3.6-pypi-postgresql #19769

00:10:52

92.0%
passed py3.6-pypi-sqlite #19767

00:10:48

91.0%
failed py3.5-pypi-postgresql #19763

00:12:08

92.0%