use latest Sphinx version for building docs

- require forked version of sphinxcontrib-versioning in CI
- reduce redundancies in .gitlab-ci.yml
- add specific docs test before deployment
14 jobs for update-sphinx in 11 minutes (queued for 3 seconds)
Status Name Job ID Coverage
  Test
passed py3.5-min-mysql #25232

00:09:19

95.0%
passed py3.5-min-postgresql #25230

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95.0%
passed py3.5-min-sqlite #25228

00:10:35

94.0%
passed py3.5-pypi-mysql #25233

00:10:57

95.0%
passed py3.5-pypi-postgresql #25241

00:08:33

95.0%
passed py3.5-pypi-sqlite #25229

00:10:51

94.0%
passed py3.6-min-mysql #25238

00:10:49

95.0%
passed py3.6-min-postgresql #25236

00:10:44

95.0%
passed py3.6-min-sqlite #25234

00:10:13

94.0%
passed py3.6-pypi-mysql #25239

00:10:53

95.0%
passed py3.6-pypi-postgresql #25237

00:10:55

95.0%
passed py3.6-pypi-sqlite #25235

00:10:42

94.0%
passed test-docs #25240

00:06:27

failed py3.5-pypi-postgresql #25231

00:00:48