[python-client] Modify python templates to resolve linting errors (#6839)

The linting results for the generated samples are as follows
where the first number is the BEFORE and the second is AFTER.

pyclient            7714 vs. 120
pyclient3           7717 vs. 120
pyclient3-asyncio   7584 vs. 120
pyclient-tornado    7633 vs. 120
pyclient3-tornado   7633 vs. 120

For the complete details please see the following gist.
https://gist.github.com/kenjones-cisco/2eb69a7e8db75e9fd53789f01570d9f2

Enforces linting for python clients by running flake8 for the generated
python client.

samples/server/petstore/flaskConnexion-python2/swagger_server/models/pet.py

@@ -6,8 +6,8 @@ from datetime import date, datetime  # noqa: F401
 from typing import List, Dict  # noqa: F401
 
 from swagger_server.models.base_model_ import Model
-from swagger_server.models.category import Category  # noqa: E501
-from swagger_server.models.tag import Tag  # noqa: E501
+from swagger_server.models.category import Category  # noqa: F401,E501
+from swagger_server.models.tag import Tag  # noqa: F401,E501
 from swagger_server import util
 
 

samples/server/petstore/flaskConnexion-python2/swagger_server/swagger/swagger.yaml

@@ -108,11 +108,11 @@ paths:
         type: "array"
         items:
           type: "string"
-          default: "available"
           enum:
           - "available"
           - "pending"
           - "sold"
+          default: "available"
         collectionFormat: "csv"
       responses:
         200:

samples/server/petstore/flaskConnexion/swagger_server/models/pet.py

@@ -6,8 +6,8 @@ from datetime import date, datetime  # noqa: F401
 from typing import List, Dict  # noqa: F401
 
 from swagger_server.models.base_model_ import Model
-from swagger_server.models.category import Category  # noqa: E501
-from swagger_server.models.tag import Tag  # noqa: E501
+from swagger_server.models.category import Category  # noqa: F401,E501
+from swagger_server.models.tag import Tag  # noqa: F401,E501
 from swagger_server import util