[Python] ImportError: cannot import name 'FileStorage' from the generated flask server

Description

When I run flask-server, I get bellow ImportError.

Traceback (most recent call last):
  File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "python-flask-server/swagger_server/__main__.py", line 3, in <module>
    import connexion
  File "python-flask-server/venv/lib/python3.6/site-packages/connexion/__init__.py", line 3, in <module>
    from .apis import AbstractAPI  # NOQA
  File "python-flask-server/venv/lib/python3.6/site-packages/connexion/apis/__init__.py", line 1, in <module>
    from .abstract import AbstractAPI  # NOQA
  File "python-flask-server/venv/lib/python3.6/site-packages/connexion/apis/abstract.py", line 14, in <module>
    from ..operation import Operation
  File "python-flask-server/venv/lib/python3.6/site-packages/connexion/operation.py", line 7, in <module>
    from .decorators import validation
  File "python-flask-server/venv/lib/python3.6/site-packages/connexion/decorators/validation.py", line 9, in <module>
    from werkzeug import FileStorage
ImportError: cannot import name 'FileStorage'

Swagger-codegen version

2.4.13

Swagger declaration file content or url

https://petstore.swagger.io/v2/swagger.json

Command line used for generation

I generated flask server from swagger-editor.

Steps to reproduce

I use venv and run swagger_server

python -m venv venv
source venv/bin/activate
pip3 install -r requirements.txt
python3 -m swagger_server

Related issues/PRs

https://github.com/zalando/connexion/issues/1149

Suggest a fix/enhancement

I can fix this bug when adding two line in reqirements.txt.

  connexion == 1.1.15
  python_dateutil == 2.6.0
  setuptools >= 21.0.0
+ Werkzeug == 0.12.2
+ flask == 0.10.1

Start with the generated Python Flask server's requirements.txt and reproduce the failure using the provided virtualenv and python3 -m swagger_server commands. Verify that the generated server starts without the FileStorage ImportError and that the dependency requirements match the supported generated code.