WIP: Retry-on-failure using requests' session adapter

[veox]

Incorporates PR #78. After that:

• Renames the parameters in retry_config:
  • forcelist -> codes
  • retries -> count
  • backoff -> backoff (no change)
• Greatly limits the default set of error codes to retry on (compared to PR #78): just 504 and 520 now.

This PR:

• Closes #66 (issue) as implemented.
• Closes #100 (PR) as conflicting.

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I think there might be issues with this approach, though:

• on every query, the adapter gets re-created - this is rather ungraceful;
• more importantly, the adapter stays with the session and is used even if retry=False is specified in a follow-up query.