Add retries to HTTP requests
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merging this one then :) 
## What does this PR do ?

Adding retries to HTTP GET & HEAD requests, when they fail with a `ConnectTimeout` exception.

The "canonical" way to do this is to use [urllib's Retry class](https://urllib3.readthedocs.io/en/latest/reference/urllib3.util.html#urllib3.util.Retry) and wrap it in a [HttpAdapter](https://requests.readthedocs.io/en/master/api/#requests.adapters.HTTPAdapter). Seems a bit overkill to me, plus it forces us to use the `requests.Session` object. I prefer this simpler implementation. I'm open to remarks and suggestions @lhoestq @yjernite 

Fixes #1102 
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