clean up unnecessary usage of the six library (awx only supports py3)

awx/api/renderers.py

@@ -5,8 +5,6 @@
 from rest_framework import renderers
 from rest_framework.request import override_method
 
-import six
-
 
 class BrowsableAPIRenderer(renderers.BrowsableAPIRenderer):
     '''
@@ -71,8 +69,8 @@ class PlainTextRenderer(renderers.BaseRenderer):
     format = 'txt'
 
     def render(self, data, media_type=None, renderer_context=None):
-        if not isinstance(data, six.string_types):
-            data = six.text_type(data)
+        if not isinstance(data, str):
+            data = str(data)
         return data.encode(self.charset)