Fix for python2.7 environment

Python envs used till now python3 for testing, now version of python
depends on the environment - for py2* python 2.7 will be used, and for
py3* appropriate python3 version will be used.

Also, added mock module for python2 envs.

tests/test_client.py

@@ -1,5 +1,8 @@
 from unittest import TestCase
-from unittest.mock import MagicMock
+try:
+    from unittest.mock import MagicMock
+except ImportError:
+    from mock import MagicMock
 
 from slack_backup import client
 from slack_backup import objects as o