Added rules for pytest fixtures:

[DKorytkin]

• rule for deprecated fixture scope as a positional argument (link)

• rule for useless pytest.mark.usefixtures for fixtures (link)

  

• added tests for these checks

• removed py27 and py35 from testing, seems not supported

[DKorytkin]

@reverbc thanks for the review, I fixed all your suggestions 👍 can you take a look one more time 😉