Fine-tune pillow atest dependencies
# See atest/README.rst for more information.
enum34; python_version < '3.0'
-pillow < 6; platform_system == 'Windows' and python_version <= '3.4'
+pillow < 7; platform_system == 'Windows' and python_version == '2.7'
+pillow < 6; platform_system == 'Windows' and python_version == '3.4'
pillow; platform_system == 'Windows' and python_version >= '3.5'
# https://github.com/IronLanguages/ironpython2/issues/113
