Fix python-fastapi free-form objects mapping and forward ref type hints

[chludwig-haufe]

This PR implements the proposed fixes for #9720 and #9721:

• Free-form object model attributes are mapped to the type hint Dict[str, Any].
• To support forward type references in the type hints generated for (recursive) model classes, add from __future__ import annotations to the model template. (This is supported in Python >= 3.7.)

With these changes in place, the generated FastAPI service could actually be started.

Python technical committee: @taxpon, @frol, @mbohlool, @cbornet, @kenjones-cisco, @tomplus, @Jyhess, @arun-nalla, @spacether

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[chludwig-haufe]

@krjakbrjak As the author of the python-fastapi generator, you might also be interested in this PR.

[krjakbrjak]

Having recursive definitions is a valid use case. LGTM. Thanks.

[wing328]

@chludwig-haufe I just spotted the following errors in the CI:

You should consider upgrading via the '/home/travis/virtualenv/python3.6.7/bin/python -m pip install --upgrade pip' command.
ImportError while loading conftest '/home/travis/build/OpenAPITools/openapi-generator/samples/server/petstore/python-fastapi/tests/conftest.py'.
tests/conftest.py:5: in <module>
    from openapi_server.main import app as application
src/openapi_server/main.py:15: in <module>
    from openapi_server.apis.pet_api import router as PetApiRouter
src/openapi_server/apis/pet_api.py:20: in <module>
    from openapi_server.models.api_response import ApiResponse
E     File "/home/travis/build/OpenAPITools/openapi-generator/samples/server/petstore/python-fastapi/src/openapi_server/models/api_response.py", line 3
E       from __future__ import annotations
E                                        ^
E   SyntaxError: future feature annotations is not defined

Can you please take a look when you've time?

[chludwig-haufe]

@wing328 I see in your command line Python 3.6. __future__.annotations requires Python >= 3.7. That's why I changed the documented requirements in 78452d6. Did I miss a file?

[krjakbrjak]

@chludwig-haufe @wing328 python 3.6 is set as default. Since the dockerfile is already generated and uses 3.7, it makes sense to build the image (which is built only when the tests pass). I opened #9771 (it addresses this issue)