refactor: replace deprecated String.prototype.substr()

[CommanderRoot]

Description

String.prototype.substr() is deprecated so we replace it with String.prototype.slice() which works similarily but isn't deprecated. .substr() probably isn't going away anytime soon but the change is trivial so it doesn't hurt to do it.

Motivation and Context

How Has This Been Tested?

I tested the return of the statement to make sure it's still the same

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation change

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read all applicable contributing documents.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [ ] I have updated the change log (if applicable).

[pygy]

Merging. Thanks for the PR @CommanderRoot!