[test] docs/requirements-py3.txt: Do not install sphinx >=1.8.0
[test] docs/requirements-py3.txt: Do not install sphinx >=1.8.0

Bug: T204189
Change-Id: I870cce4fcce157629b2238d178bcbde6ed5820b5

PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core