MAINT: Pin scikit-image under 0.17 to avoid requiring Cython
MAINT: Pin scikit-image under 0.17 to avoid requiring Cython

Also, remove leftovers of dependencies pinned for Python < 3.6 (#486)
oesteban/niworkflows,poldracklab/niworkflows,poldracklab/niworkflows,oesteban/niworkflows,oesteban/niworkflows