Update Pillow used by atests to more secure one.
Problem reported by dependabot. It doesn't really affect us, though.
enum34; python_version < '3.0'
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'
+pillow >= 7.1.0; platform_system == 'Windows' and python_version >= '3.5'
# https://github.com/IronLanguages/ironpython2/issues/113
docutils >= 0.9; platform_python_implementation != 'IronPython'
