ci: separate cache for different db backends

12 jobs for 45-fix-failing-builds in 13 minutes and 21 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
passed py3.5-min-mysql #19938

00:09:17

92.0%
passed py3.5-min-postgresql #19936

00:10:05

92.0%
passed py3.5-min-sqlite #19934

00:10:58

91.0%
passed py3.5-pypi-mysql #19939

00:10:16

92.0%
passed py3.5-pypi-postgresql #19937

00:10:16

92.0%
passed py3.5-pypi-sqlite #19935

00:13:17

91.0%
passed py3.6-min-mysql #19944

00:09:26

92.0%
passed py3.6-min-postgresql #19942

00:09:29

92.0%
passed py3.6-min-sqlite #19940

00:08:32

91.0%
passed py3.6-pypi-mysql #19945

00:09:52

92.0%
passed py3.6-pypi-postgresql #19943

00:09:37

92.0%
passed py3.6-pypi-sqlite #19941

00:08:38

91.0%