Add *.bak for gitignore

[amitvadhel]

While running python-modernize, its generate .bak file.
So, I guess we need to ignore *.bak in git

@edx/ecommerce

[openedx-webhooks]

Thanks for the pull request, @amitvadhel! I've created OSPR-3670 to keep track of it in JIRA. JIRA is a place for product owners to prioritize feature reviews by the engineering development teams.

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[openedx-webhooks]

@amitvadhel 🎉 Your pull request was merged!

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