Isolate redis-entraid dependency for tests

[akx]

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?

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Description of change

This PR follows up on #3502 (comment).

It:

• moves the duplicated redis-entraid specific testing code to a separate module
• defers the imports for redis-entraid (so tests can be run even without redis-entraid installed; given redis-entraid is currently unlicensed, having it as a required dependency is problematic)
• fixes the CI to uninstall the PyPI redis-py version that installing redis-entraid installs, and instead installs the working copy version as editable (so e.g. metadata.version() returns the correct version).

[petyaslavova]

Hi @akx thank you for your contribution! We’ll review your change soon.

[akx]

@petyaslavova @vladvildanov Rebased post #3518.

[petyaslavova]

LGTM.

[petyaslavova]

It's great that the duplicated code has been extracted into a separate file—it will make maintenance and future extensions easier! 🙂 However, I have some concerns about some of the other changes in this PR. Please see my inline comments.