Add minor changes in jupyter_drives in base.py and in tests/test_handlers.py

[HaudinFlorence]

This PR introduces these 2 changes:

• in base.py, replace os.path_exists by os.path.exists
• in test_handlers.py, replace mock.s3 by mock_aws

[github-actions[bot]]

:point_left: Launch a Binder on branch _HaudinFlorence/jupyter-drives/add_a_minor_change_in_base_pythonfile