Remove six usage in ValidationHasMetadataRule.py

The python 2.x support has been dropped wo we don't need it anymore.

Signed-off-by: Gael Chamoulaud (Strider) <gchamoul@redhat.com>
Change-Id: I6a5971413852ed94407704285f071dee96302b1f

.ansible-lint_rules/ValidationHasMetadataRule.py

@@ -1,5 +1,4 @@
 import os
-import six
 import yaml
 
 from ansiblelint import AnsibleLintRule
@@ -105,8 +104,7 @@ class ValidationHasMetadataRule(AnsibleLintRule):
                                 {path: data},
                                 '*metadata* should contain a %s key' % info))
                             continue
-                        if not isinstance(metadata.get(info),
-                                          six.string_types):
+                        if not isinstance(metadata.get(info), str):
                             results.append((
                                 {path: data},
                                 '*%s* should be a string' % info))