Use OAS 2 -> 3 converter in 'httpdomain' renderer

This is the last crucial piece of supporting OAS 2 in the new renderer.
With that support landed, couple of cool features previously available
for OAS 3 only is now supported for OAS 2.

For examples:

 * The new renderer can render example snippets for OAS 2 now based on
   'schema.example' property.

 * The new renderer can generate example based on schema for OAS 2 too.

Author: ikalnytskyi

sphinxcontrib/openapi/renderers/_httpdomain.py

@@ -10,6 +10,7 @@
 import requests
 import sphinx.util.logging as logging
 
+from sphinxcontrib.openapi import _lib2to3 as lib2to3
 from sphinxcontrib.openapi.renderers import abc
 from sphinxcontrib.openapi.schema_utils import example_from_schema
 
@@ -153,6 +154,9 @@ def __init__(self, state, options):
 
     def render_restructuredtext_markup(self, spec):
         """Spec render entry point."""
+
+        if spec.get("swagger") == "2.0":
+            spec = lib2to3.convert(spec)
         yield from self.render_paths(spec.get("paths", {}))
 
     def render_paths(self, paths):