[CIVIS-1683][CIVIS-5258] FIX responses are no longer mutable; support snake_case key access for nested responses

[jacksonlee-civis]

This PR re-implements the civis.response.Response class, in a bid to address two long-standing issues:

• Resolves Make the Response object immutable #228. The Response class used to inherit from dict, leading to mutability issues. The new Response class from this PR explicitly bans the "setitem" and "setattr" operations.
• Resolves Inconsistent snake_case vs camelCase for Response keys #317. Despite what I said in this comment, I've decided to allow both snake_case and camelCase key access for nested Response objects.

• (For Civis employees only) Reference to a relevant ticket in the pull request title
• Changelog entry added to CHANGELOG.md at the repo's root level
• Description of change in the pull request description
• If applicable, unit tests have been added and/or updated
• The CircleCI builds have all passed

[margaretott]

LGTM