Retry GET requests

[ttung]

1. Add a retry policy class that dictates which requests are retried and how often they are retried.
2. Add a test that uses the fake 504 mechanism in data-store to test the retry policy.

Connects to HumanCellAtlas/data-store#485

[codecov-io]

Codecov Report

Merging #87 into master will decrease coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   87.66%   87.54%   -0.13%     
==========================================
  Files          28       28              
  Lines         924      931       +7     
==========================================
+ Hits          810      815       +5     
- Misses        114      116       +2

Impacted Files	Coverage Δ
hca/util/__init__.py	90.2% <100%> (-0.98%)	⬇️
hca/dss/upload_to_cloud.py	70.58% <0%> (+1.96%)	⬆️

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[hannes-ucsc]

Let me plug https://github.com/HumanCellAtlas/data-store/blob/master/dss/util/retry.py#L14 here which could also be used for this if it weren't in a different project.

[ttung]

Let me plug https://github.com/HumanCellAtlas/data-store/blob/master/dss/util/retry.py#L14 here which could also be used for this if it weren't in a different project.

time for HumanCellAtlas/dcp-py-common

[kislyuk]

urllib3 has retry logic built in. This retry policy logic can be customized and accessed via requests via the max_retries kwarg, to which you can pass a urllib3.Retry() object, which you can configure or subclass to customize this behavior.

Let's switch to using that functionality.

[ttung]

Updated to use urllib3 retry.

[kislyuk]

LGTM