Closeio

What's Changed
* Update client to respect the newer `RateLimit` header by AlecRosenbaum in https://github.com/closeio/closeio-api/pull/115


**Full Changelog**: https://github.com/closeio/closeio-api/compare/v2.0...v2.1

Changes in this release:
* update 429 handling to use response headers rather than the json body
* **(Breaking Change)** Drop python 2 support
* Other minor or project-internal changes (update pytest, test on more recent python 3 versions, etc)

This release fixes how the client handles 204 API responses (108).

This PR adds retries functionality for 502, 503, and 504 status codes

This release changes the base URL for the Close API from `https://app.close.io/api/v1/` to `https://api.close.com/api/v1/`

We're happy to publish a stable v1.0 of our Close.io API wrapper! The main highlights of this release are:
* Significantly improved code quality, clarity, and documentation.
* Automatic retrying of rate-limited requests.
* **Breaking change:** Removal of the asynchronous code. If you relied on our `async=True` flag in the past, you'll have to refactor your application. You can choose to run multiple concurrent `api.get/post/put/delete` requests via `threading`, `multiprocessing`, `concurrent.futures`, `gevent`, etc., depending on your specific use case and environment.