Fix rate limiting backoff behavior

[AlecRosenbaum]

This change:

• drops python 2 (so I can use contextlib)
• bumps the major version of the library
• adds testing on python versions 3.7-3.10
• updates the images to not use the deprecated circleci/ images, instead preferring cimg/
• fixes rate limit backoff logic to read headers instead of the json body
• bumps the pinned responses version, so headers can be added to mocked responses
• bumps the pinned requests and pytest versions as well

Once this merges, a new release should be published to PyPi