Refresh pass quick wins

[joshrotenberg]

Summary

• fix Active-Active region deletion to send its JSON request body and add a regression test
• refresh stale README install snippets and Python env-var guidance
• sync the Python crate version/lockfile with the Rust crate and verify the packaged extension path

Verification

• cargo test --test subscriptions_tests
• cargo test --manifest-path python/Cargo.toml
• python3 -m venv /tmp/redis-cloud-pytest && source /tmp/redis-cloud-pytest/bin/activate && python -m pip install -q maturin pytest pytest-asyncio && cd python && maturin develop -q && pytest tests/test_client.py

Closes #58
Closes #59
Closes #60
Partially addresses #68
Refs #70