add equality and hashability to Retry and backoff classes

[terencehonles]

Description of change

This change adds equality and hashability to Retry and backoff classes.

This broke in DjangoRQ (fixed) after #3622 / #3614 and I provided a workaround in rq/django-rq#706, but this allows other downstream libraries to compare two retry instances.

Pull Request check-list

Please make sure to review and check all of these items:

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

[Copilot]

Pull Request Overview

This PR adds equality and hashability methods to the Retry and various backoff classes to support reliable comparisons between retry instances in downstream libraries.

• Introduces eq and hash for Retry and multiple backoff classes.
• Augments the test suite with new test cases to validate equality/hashability behavior.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
tests/test_retry.py	Adds tests validating equality and hashability of Retry instances.
redis/retry.py	Implements eq and hash methods for the Retry class.
redis/backoff.py	Adds equality and hashability implementations for several backoff types.