Add more tests around retry logic

[lightswitch05]

Why

I'm interested in this project and figured writing tests was a good opportunity to learn more about it. I saw code coverage was not at 100% - and so I focused on tests that would improve the coverage.

What

• Write a test for the retry logic on connection error, with eventual success
• Write a test for the retry logic on connection error - where max attempts are reached
• Write a test for the retry logic on 5XX errors - where max attempts are reached
• All of these tests are rather slow, so I mocked time.sleep to speed them up. Since that had good results, I tweaked the rate limiting test to also mock time.sleep - speeding up the tests.

Testing

• Run the tests - verify they are not really slow
• I used my other PR - https://github.com/closeio/closeio-api/pull/116 - to check the coverage of the new tests. These additional tests bump up the total coverage from 76% to 88%. There are opportunities for further improvement by removing some unreachable/dead code, but I didn't want to make any library changes.

[lightswitch05]

Hey @AlecRosenbaum - any interest in this PR?