FIX-#6952: use render_as_string to get sqlalchemy engine url

[tochigiv]

What do these changes do?

Instead of calling url.__str__(), call url.render_as_string() directly.

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• passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py
• passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py
• signed commit with git commit -s
• Resolves BUG: Worker nodes are given incorrect credentials from SQLAlchemy object URL #6952
• tests added and passing
• module layout described at docs/development/architecture.rst is up-to-date

[YarShev]

Hi @tochigiv, thanks for the contribution. Good to see that the changes fix the issue but left a question.

[sfc-gh-dpetersohn]

Thanks @tochigiv , LGTM!

[YarShev]

@tochigiv, LGTM, thanks!