Inherit TerraformJSONBase from Mapping ABC (#62)

Inhering from collections.abc.Mapping gives us __contains__, keys,
items, values, get, __eq__, and __ne__ for free. It also makes objects
behave more dict-like

tftest.py

@@ -154,7 +154,7 @@ def parse_args(init_vars=None, tf_vars=None, targets=None, **kw):
   return cmd_args
 
 
-class TerraformJSONBase(object):
+class TerraformJSONBase(collections.abc.Mapping):
   "Base class for JSON wrappers."
 
   def __init__(self, raw):
@@ -163,6 +163,12 @@ def __init__(self, raw):
   def __bytes__(self):
     return bytes(self._raw)
 
+  def __getitem__(self, index):
+    return self._raw[index]
+
+  def __iter__(self):
+    return iter(self._raw)
+
   def __len__(self):
     return len(self._raw)
 
@@ -187,12 +193,6 @@ def __getattr__(self, name):
   def __getitem__(self, name):
     return self._raw[name].get('value')
 
-  def __contains__(self, name):
-    return name in self._raw
-
-  def __iter__(self):
-    return iter(self._raw)
-
 
 class TerraformPlanModule(TerraformJSONBase):
   "Minimal wrapper for parsed plan output modules."
@@ -218,12 +218,6 @@ def resources(self):
                              for res in self._raw.get('resources', []))
     return self._resources
 
-  def __getitem__(self, name):
-    return self._raw[name]
-
-  def __contains__(self, name):
-    return name in self._raw
-
 
 class TerraformPlanOutput(TerraformJSONBase):
   "Minimal wrapper for Terraform plan JSON output."