tests: bump pytest version requirement to allow 3.x

pytest is an active project, since our previous requirement setting there
have been many releases, with new features and bug fixes.

One of the improvements in pytest-3.0 is that 'yield_fixture' (a fixture
that can embed cleanup code after a yield) is no longer different than
'fixture', i.e. a standard fixture supports the yield cleanup mechanism. We
will switch to use that when we make 3.0 mandatory.