Support Hiredis >= 1.x only (remove packaging dependency)

[akx]

Pull Request check-list

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• Does $ tox pass with this change (including linting)?
• 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?
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Description of change

This PR:

• disables hiredis use if hiredis is < 1.0 (i.e. if the version starts with 0.x). This shouldn't be a problem, since Hiredis 1.0 was released in 2019, and the [hiredis] extra has required hiredis >= 1.0 since 4.0 (866ac00).
• with that, it removes the runtime dependency on packaging, which was only used to determine whether the version of Hiredis in use is recent enough.
• removes the compatibility code that was required with Hiredis < 1.0.