packaging: run sphinx on py3 by default

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branch : stable
8 jobs for topic/stable/py3-docs-2 in 5 minutes and 28 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #123991
checks-py2

00:02:48

passed #123992
checks-py3

00:00:51

passed #123997
doc

00:00:36

manual #123990
allowed to fail manual
pytype
passed #123993
tests-py2-cext

00:04:37

passed #123994
tests-py2-pure

00:05:27

passed #123995
tests-py3-cext

00:03:55

passed #123996
tests-py3-pure

00:05:28