tests: fix hanging doc building tests

sphinx-contrib-versioning seems to be not yet compatible with
Sphinx>2.0. Pin Sphinx as long as build of documentation keeps hanging.
14 jobs for fix-failing-doc-test in 11 minutes and 31 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Test
passed py3.5-min-mysql #44847

00:11:08

passed py3.5-min-postgresql #44845

00:09:20

passed py3.5-min-sqlite #44843

00:09:16

passed py3.5-pypi-mysql #44848

00:10:07

passed py3.5-pypi-postgresql #44846

00:10:12

passed py3.5-pypi-sqlite #44844

00:09:37

passed py3.6-min-mysql #44853

00:09:59

passed py3.6-min-postgresql #44851

00:09:45

passed py3.6-min-sqlite #44849

00:09:11

passed py3.6-pypi-mysql #44854

00:10:38

passed py3.6-pypi-postgresql #44852

00:09:57

passed py3.6-pypi-sqlite #44850

00:09:30

passed test-docs #44855

00:04:22

 
  Coverage
passed global-coverage #44856

00:00:22

95.0%