tests: temporarily pin pytest to fix AppVeyor builds
tests: temporarily pin pytest to fix AppVeyor builds

pfmoore/pip,pypa/pip,xavfernandez/pip,pypa/pip,sbidoul/pip,rouge8/pip,rouge8/pip,pradyunsg/pip,sbidoul/pip,pradyunsg/pip,xavfernandez/pip,xavfernandez/pip,pfmoore/pip,rouge8/pip