[python-flask] adopt python3.5+ syntax (#16375)

* adopt python3.5+ syntax

removing some residual python2 code, since it is not supported anymore, like:
- no need for `six` anymore
- no need for encoding utf8 in top file
- remove `object` inheritance in base model
- remove absolute import `__future__`

* generate samples

applying the new templates

* update python ignore pattern

ignore all the `.venv` folders

samples/server/petstore/python-flask/openapi_server/test/test_pet_controller.py

@@ -1,10 +1,6 @@
-# coding: utf-8
-
-from __future__ import absolute_import
 import unittest
 
 from flask import json
-from six import BytesIO
 
 from openapi_server.models.api_response import ApiResponse  # noqa: E501
 from openapi_server.models.pet import Pet  # noqa: E501
@@ -20,23 +16,7 @@ class TestPetController(BaseTestCase):
 
         Add a new pet to the store
         """
-        body = {
-  "photoUrls" : [ "photoUrls", "photoUrls" ],
-  "name" : "doggie",
-  "id" : 0,
-  "category" : {
-    "name" : "name",
-    "id" : 6
-  },
-  "tags" : [ {
-    "name" : "name",
-    "id" : 1
-  }, {
-    "name" : "name",
-    "id" : 1
-  } ],
-  "status" : "available"
-}
+        body = {"photoUrls":["photoUrls","photoUrls"],"name":"doggie","id":0,"category":{"name":"name","id":6},"tags":[{"name":"name","id":1},{"name":"name","id":1}],"status":"available"}
         headers = { 
             'Content-Type': 'application/json',
             'Authorization': 'Bearer special-key',
@@ -71,7 +51,7 @@ class TestPetController(BaseTestCase):
 
         Finds Pets by status
         """
-        query_string = [('status', 'available')]
+        query_string = [('status', ['status_example'])]
         headers = { 
             'Accept': 'application/json',
             'Authorization': 'Bearer special-key',
@@ -124,23 +104,7 @@ class TestPetController(BaseTestCase):
 
         Update an existing pet
         """
-        body = {
-  "photoUrls" : [ "photoUrls", "photoUrls" ],
-  "name" : "doggie",
-  "id" : 0,
-  "category" : {
-    "name" : "name",
-    "id" : 6
-  },
-  "tags" : [ {
-    "name" : "name",
-    "id" : 1
-  }, {
-    "name" : "name",
-    "id" : 1
-  } ],
-  "status" : "available"
-}
+        body = {"photoUrls":["photoUrls","photoUrls"],"name":"doggie","id":0,"category":{"name":"name","id":6},"tags":[{"name":"name","id":1},{"name":"name","id":1}],"status":"available"}
         headers = { 
             'Content-Type': 'application/json',
             'Authorization': 'Bearer special-key',

samples/server/petstore/python-flask/openapi_server/test/test_store_controller.py

@@ -1,10 +1,6 @@
-# coding: utf-8
-
-from __future__ import absolute_import
 import unittest
 
 from flask import json
-from six import BytesIO
 
 from openapi_server.models.order import Order  # noqa: E501
 from openapi_server.test import BaseTestCase
@@ -52,7 +48,7 @@ class TestStoreController(BaseTestCase):
             'Accept': 'application/json',
         }
         response = self.client.open(
-            '/v2/store/order/{order_id}'.format(order_id=1),
+            '/v2/store/order/{order_id}'.format(order_id=56),
             method='GET',
             headers=headers)
         self.assert200(response,

samples/server/petstore/python-flask/openapi_server/test/test_user_controller.py

@@ -1,10 +1,6 @@
-# coding: utf-8
-
-from __future__ import absolute_import
 import unittest
 
 from flask import json
-from six import BytesIO
 
 from openapi_server.models.user import User  # noqa: E501
 from openapi_server.test import BaseTestCase