[tests] Remove cffi from dev-requirements.txt
[tests] Remove cffi from dev-requirements.txt

T286457 was solved upstream:
https://foss.heptapod.net/pypy/cffi/-/issues/503

Bug: T286457
Change-Id: I8d1d9be9453f81f162fb3e5a9d008ff0e05bea72

wikimedia/pywikibot-core,wikimedia/pywikibot-core