Replace usage of mock with unittest.mock (#1054)

Since Python 3, the mock dependency is no more required as
it is already part of the unittest module.

tests/test_auth.py

@@ -1,5 +1,5 @@
 """HTTP authentication-related tests."""
-import mock
+from unittest import mock
 import pytest
 
 from httpie.plugins.builtin import HTTPBasicAuth