fix(tests): pin pytest-xdist ~=1.28 so that 2.0 is not installed
fix(tests): pin pytest-xdist ~=1.28 so that 2.0 is not installed

seems like xdist 2.0 has consistently broken our builds on both Travis
and Circle

oesteban/niworkflows,poldracklab/niworkflows,oesteban/niworkflows,poldracklab/niworkflows,oesteban/niworkflows