feat: add before_sleep as constructor arg to RetryStrategy.

[rwiker]

Issue

N/A

Description

For the uploader, we want to capture the number of retries for uploads. We can do that through the before_sleep argument to tenacity.Retrying / tenacity.AsyncRetrying: we can pass a closure that modifies a dict/list/object`.

How to test

The default value for this argument is the same as what we previously hardcoded, so there should be no difference in behaviour.

Checklist

• No redundant `print()` statements, commented-out code, or other remnants from the development 👀
• New/refactored code is following same conventions as the rest of the code base 🧬
• New/refactored code is tested ⚙
• Documentation has been updated 🧾
• Commits are semantic ✅