FIX-#7379: Fix __imul__ performing addition instead of multiplication

[noloerino]

What do these changes do?

• first commit message and PR title follow format outlined here

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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: __imul__ uses __add__ instead of __mul__ #7379
• tests added and passing
• module layout described at docs/development/architecture.rst is up-to-date