"Freeze" the openapi version to 3.0.2

[aldbr]

• FastAPI 0.99.0 is using openapi 3.1.0 by default: https://fastapi.tiangolo.com/release-notes/?h=0.99.0#features_3
• Autorest does not support it yet

error   | schema_violation | Schema violation: must match pattern "^3\.0\.\d(-.+)?$" (openapi)
  pattern: ^3\.0\.\d(-.+)?$
    - file:///tmp/pytest-of-aldbr/pytest-49/test_regenerate_client0/openapi.json:1:2