[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/client/petstore/python/tests/test_api_client.py

@@ -1,5 +1,7 @@
 # coding: utf-8
 
+# flake8: noqa
+
 """
 Run the tests.
 $ pip install nose (optional)

samples/client/petstore/python/tests/test_api_exception.py

@@ -1,5 +1,7 @@
 # coding: utf-8
 
+# flake8: noqa
+
 """
 Run the tests.
 $ pip install nose (optional)

samples/client/petstore/python/tests/test_deserialization.py

@@ -1,5 +1,7 @@
 # coding: utf-8
 
+# flake8: noqa
+
 """
 Run the tests.
 $ pip install nose (optional)

samples/client/petstore/python/tests/test_enum_arrays.py

@@ -1,5 +1,7 @@
 # coding: utf-8
 
+# flake8: noqa
+
 """
 Run the tests.
 $ pip install nose (optional)

samples/client/petstore/python/tests/test_map_test.py

@@ -1,5 +1,7 @@
 # coding: utf-8
 
+# flake8: noqa
+
 """
 Run the tests.
 $ pip install nose (optional)

samples/client/petstore/python/tests/test_order_model.py

@@ -1,5 +1,7 @@
 # coding: utf-8
 
+# flake8: noqa
+
 """
 Run the tests.
 $ pip install nose (optional)

samples/client/petstore/python/tests/test_pet_api.py

@@ -1,5 +1,7 @@
 # coding: utf-8
 
+# flake8: noqa
+
 """
 Run the tests.
 $ pip install nose (optional)

samples/client/petstore/python/tests/test_pet_model.py

@@ -1,5 +1,7 @@
 # coding: utf-8
 
+# flake8: noqa
+
 """
 Run the tests.
 $ pip install nose (optional)

samples/client/petstore/python/tests/test_store_api.py

@@ -1,5 +1,7 @@
 # coding: utf-8
 
+# flake8: noqa
+
 """
 Run the tests.
 $ pip install nose (optional)

samples/client/petstore/python/tests/util.py

@@ -1,3 +1,5 @@
+# flake8: noqa
+
 import random