Bump dependency on six from 1.8.0 to 1.9.0

cwltool itself doesn't depend on six 1.9.0, but prov does, and having
matching dependencies helps Pip find a correct set of packages to
install (see common-workflow-language#872).

setup.py

@@ -56,7 +56,7 @@
           'shellescape >= 3.4.1, < 3.5',
           'schema-salad >= 2.7.20180719125426, < 3',
           'mypy-extensions',
-          'six >= 1.8.0',
+          'six >= 1.9.0',
           'psutil',
           'prov == 1.5.1',
           'bagit >= 1.6.4',