dev-requirements.txt: Update pytest requirement
dev-requirements.txt: Update pytest requirement

The new release of pytest-timeout requires pytest>=3.6.0.

Bug: T197202
Change-Id: I4ef6c2c16e36e24d5b57c10b09fd61bfbe757539

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