fix: drop six dependency (#1452)

Fixes #1446 🦕
diff --git a/googleapiclient/schema.py b/googleapiclient/schema.py
index 00f8588..95767ef 100644
--- a/googleapiclient/schema.py
+++ b/googleapiclient/schema.py
@@ -57,7 +57,6 @@
 The constructor takes a discovery document in which to look up named schema.
 """
 from __future__ import absolute_import
-import six
 
 # TODO(jcgregorio) support format, enum, minimum, maximum
 
@@ -255,7 +254,7 @@
             if "properties" in schema:
                 properties = schema.get("properties", {})
                 sorted_properties = OrderedDict(sorted(properties.items()))
-                for pname, pschema in six.iteritems(sorted_properties):
+                for pname, pschema in sorted_properties.items():
                     self.emitBegin('"%s": ' % pname)
                     self._to_str_impl(pschema)
             elif "additionalProperties" in schema: