Replace unmaintained `backoff` with another option

[gonsoos]

"backoff" is unmaintained upstream and incompatible with py3.14+, see

• Maintenance help? litl/backoff#211 (which hasn't made progress since May)
• Usage of deprecated asyncio.iscoroutinefunction()
  https://github.com/litl/backoff/blob/d82b23c42d7a7e2402903e71e7a7f03014a00076/backoff/_decorator.py#L101
  https://docs.python.org/3.14/whatsnew/3.14.html#pending-removal-in-python-3-16
• Reliance on implicit creation of event loop, which has been removed in Python 3.14
  https://github.com/litl/backoff/blob/d82b23c42d7a7e2402903e71e7a7f03014a00076/tests/test_backoff_async.py#L695
  https://docs.python.org/3.14/whatsnew/3.14.html#id7
• https://bugs.gentoo.org/958592

A maintained alternative would be for example https://github.com/jd/tenacity.

[xsuchy]

The first issue has comments offering help. I will wait some time to make conclusions.

[pastalian]

https://github.com/litl/backoff has just been archived, and my attempt to reach the fork maintainer hasn't been successful so far.