py.test to 4

[vegu]

Currently running pytest>=2.9.2, <4 - might be good to push that to 4 at some point, will need some refactoring of test code though, a quick test run threw up some errors at least.

[vegu]

@grizz i know you were in the middle of doing this with changes to pytest-filedata, i've pinned pytest-filedata to 0.3 for now, just so we can get tests to pass until vaping is adjusted to the changes in 0.4