[CIVIS-2564] ENH raise a warning for deprecated API endpoint methods

After this pull request, a FutureWarning is raised when we call a deprecated API endpoint method.

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

civisanalytics / civis-python