Update pillow version to <8.4.0, to keep support for Python 3.6.x

Author: melissalinkert

setup.py

@@ -81,7 +81,7 @@ def read(fname):
       setup_requires=['flake8'],
       install_requires=[
           'click==7.0',
-          'pillow==7.1.0',
+          'pillow<8.4.0',
           'numpy==1.17.3',
           'zarr==2.8.1',
           'kajiki==0.8.2',