feat: add wait_fibonacci strategy

[samuelefiorini]

Summary

Adds a new wait_fibonacci wait strategy that increases retry delays following the Fibonacci sequence (1s, 2s, 3s, 5s, 8s, 13s, …), capped by an optional max parameter.

Fibonacci backoff is a compromise between linear and exponential backoff — it ramps up more gently than exponential, making it a good fit for scenarios where a resource might be temporarily unavailable and you want to reduce latency without overwhelming the server.

Example

@retry(wait=wait_fibonacci(max=60))
def call_flaky_service():
    ...

Design notes

• Stateful: unlike most wait strategies that compute from retry_state.attempt_number, wait_fibonacci maintains an internal sequence. This is called out in the docstring with a .. note:: directive.
• max parameter supports both int/float and timedelta, consistent with other wait strategies.
• OverflowError handling: for extremely long retry sequences, an OverflowError is caught and the max value is returned.

Checklist

• Implementation in tenacity/wait.py
• Exported in __init__.py and __all__
• Tests in tests/test_tenacity.py (4 test methods following existing patterns)
• Documentation in doc/source/index.rst
• Release note
• All checks pass (poe all — linting, formatting, mypy, pytest, doctests, docs build)