salt/output/__init__.py: basestring => six.string_types

************* Module salt.output
salt/output/__init__.py:129: [W1602(basestring-builtin), strip_esc_sequence] basestring built-in referenced
Merge pull request #17742 from hulu/output-string-types

salt/output/__init__.py: basestring => six.string_types
Merge branch 'develop' into with-pylint-pragmas