Update renderers/_httpdomain.py for recent Python versions

At some point, apparently Python 3.3, collections.Mapping moved to
collections.abc.Mapping [0]. Update the imports accordingly.

[0] https://docs.python.org/3/library/collections.abc.html

Co-authored-by: Quentin Monnet <quentin@isovalent.com>

sphinxcontrib/openapi/renderers/_httpdomain.py

@@ -1,6 +1,7 @@
 """OpenAPI spec renderer."""
 
 import collections
+import collections.abc
 import copy
 import functools
 import http.client
@@ -174,7 +175,7 @@ def _get_schema_type(schema):
 
 
 _merge_mappings = deepmerge.Merger(
-    [(collections.Mapping, deepmerge.strategy.dict.DictStrategies("merge"))],
+    [(collections.abc.Mapping, deepmerge.strategy.dict.DictStrategies("merge"))],
     ["override"],
     ["override"],
 ).merge