[Python] Generated tests doesn't include required models properties

Description

The generated tests fail when a model has some required properties (e.g. https://travis-ci.org/FlatIO/api-client-python/jobs/249376469)

Swagger-codegen version

master (e50a4cca36)

Swagger declaration file content or url

  AssignmentCopy:
    description: Assignment copy operation
    type: object
    required:
    - classroom
    properties:
      classroom:
        type: string
        description: The destination classroom where the assignment will be copied

Command line used for generation

java -jar swagger-codegen-cli.jar generate -i https://flatio.github.io/api-reference/swagger.yaml -l python -c python-flat.json

{
  "packageName": "flat_api",
  "packageVersion": "0.3.0",
  "packageUrl": "https://github.com/FlatIO/api-client-python"
}

Steps to reproduce

$ nosetests
======================================================================
ERROR: Test AssignmentCopy
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/FlatIO/api-client-python/test/test_assignment_copy.py", line 38, in testAssignmentCopy
    model = flat_api.models.assignment_copy.AssignmentCopy()
  File "/home/travis/build/FlatIO/api-client-python/flat_api/models/assignment_copy.py", line 48, in __init__
    self.classroom = classroom
  File "/home/travis/build/FlatIO/api-client-python/flat_api/models/assignment_copy.py", line 71, in classroom
    raise ValueError("Invalid value for `classroom`, must not be `None`")
ValueError: Invalid value for `classroom`, must not be `None

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