Improve the Python generator

[mridang]

1. The API methods don't have return types.
2. There are many redundant methods which should be remove e.g https://github.com/OpenAPITools/openapi-generator/blob/v7.12.0/modules/openapi-generator/src/main/resources/python/api.mustache#L48 https://github.com/OpenAPITools/openapi-generator/blob/v7.12.0/modules/openapi-generator/src/main/resources/python/api.mustache#L62
3. When x-is-empty-body is specified, then the var should be None
4. In some places, a "none_type" is used. This is not valid and we should define none_type = type(None)
5. Methods such as *_serialize don't have any parameter types

[mridang]

Addressed in #35. The Python generator now: (1) emits return types on all API methods (e.g. -> Pet, -> bytes); (2) generates only the two intended per-operation methods (op + op_with_http_info) — the redundant upstream _serialize/_without_preload_content variants are gone; (3) maps an empty required body to None; (4) no longer uses the invalid none_type (zero references remain); (5) carries type annotations throughout (pydantic v2 models, typed helpers).