[python] fixes addProps sometime breaks
Verification test added for model and property
This happened because schema baseNames were colliding when an items schema and an additionalProperties schema had identical schemas. When that happened, one (earlier processed) codegenProperty was returned from a cache.
I added the methods getAdditionalPropertiesName and getItemsName to provide these names and prevent collisions

Synonym PR for openapi-json-schema-tools/openapi-json-schema-generator#93

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