refactor: Replace mock with unittest.mock

[torresxb1]

Note: this won't pass on Python 2 because of the mock -> unittest.mock changes. I guess this other PR (where python 2 testing is removed) has to be merged first:
#2182

Issue #, if available:

Description of changes:

• replace mock imports with unittest.mock (part of python 2 deprecation, mock is part of the standard library as unittest.mock from Python 3.3 onwards)
• fix a deprecation warning

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.

[torresxb1]

removing package, since this PR replaces all uses

[torresxb1]

should now be handled by merged PR: #2299