Fix Nose deprecation warning with Pytest

[kuwv]

Pytest had builtin support for nose that is not deprecated in 7.0. The current tests used by invoke utitlize a setup() method which Pytest marks as Nose compatibility. Changing this method to setup_method() allows the tests to succeed without warnings.

[kuwv]

#903

[bitprophet]

Right, and I think I eventually updated pytest-relaxed to account for setup_method style setups too. If I'm right about that, this shouldn't be a problem. Let's roll this into the PR.