[Python] Add missing Mock library for Python 2.7. (#10939)

* [Python] Add missing Mock library for Python 2.7.

* Update template for test-requirements.txt.

modules/openapi-generator/src/main/resources/python-legacy/test-requirements.mustache

@@ -9,4 +9,9 @@ randomize>=0.13
 pytest~=4.6.7 # needed for python 2.7+3.4
 pytest-cov>=2.8.1
 pytest-randomly==1.2.3 # needed for python 2.7+3.4
-{{/useNose}}
+{{/useNose}}
+{{^asyncio}}
+{{^tornado}}
+mock; python_version<'3.0'
+{{/tornado}}
+{{/asyncio}}

samples/client/petstore/python-legacy/test-requirements.txt

@@ -1,3 +1,4 @@
 pytest~=4.6.7 # needed for python 2.7+3.4
 pytest-cov>=2.8.1
 pytest-randomly==1.2.3 # needed for python 2.7+3.4
+mock; python_version<'3.0'

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

@@ -13,7 +13,12 @@ $ nosetests -v
 
 import os
 import unittest
-from unittest.mock import patch
+
+try:
+    from unittest.mock import patch
+except ImportError:
+    # Python 2.7
+    from mock import patch
 
 import petstore_api
 from petstore_api import Configuration

samples/openapi3/client/petstore/python-legacy/test-requirements.txt

@@ -1,3 +1,4 @@
 pytest~=4.6.7 # needed for python 2.7+3.4
 pytest-cov>=2.8.1
 pytest-randomly==1.2.3 # needed for python 2.7+3.4
+mock; python_version<'3.0'