dev-requirements.txt: Force installing Flask<1.0.1
dev-requirements.txt: Force installing Flask<1.0.1

Bug: T193383
Change-Id: I217d7d559ecdc9cf897e20a4ddb763a9234dbeb6

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