Clean up Py2 dependencies and update to use native unittest.mock

[hawflau]

Issue #, if available:

Description of changes:

• Remove all Py2 dependencies
• Update to use native unittest.mock instead of external mock library

Description of how you validated changes:

Checklist:

• Add/update tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected

Examples?

Please reach out in the comments, if you want to add an example. Examples will be
added to sam init through https://github.com/awslabs/aws-sam-cli-app-templates/

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jfuss]

Do we still need six if we are on py2?

[hawflau]

We only use string_types and assertCountEqual from six. Let me check if we can safely remove them and replace with something else. If yet, I'll submit another PR.

[jfuss]

Let's do another PR to keep things scoped.