Add OpenAPI Specification v3.1 objects (#41)

* Add OpenAPI Specification v3.1 objects

* Update docstrings

* Remove pattern_properties from Schema

* Fix Schema and MediaType models

Remove example from MediaType
Remove nullable from Schema
Add support list in type in Schema
Add content_encoding to Schema
Add content_media_type to Schema

Author: tyzhnenko

.flake8

@@ -6,4 +6,5 @@ max-complexity = 18
 per-file-ignores =
     tests/test_v2.py: E501
     tests/test_v3.py: E501
+    tests/test_v31.py: E501
 select = B,C,E,F,W,T4,B9

openapidocs/v3.py

@@ -1,7 +1,7 @@
 """
 This module defines classes that can be used to generate OpenAPI Documentation
 version 3.
-https://swagger.io/specification/
+https://swagger.io/specification/v3/
 """
 
 from abc import ABC