Upgrade to use fstring

[daico007]

PR Summary:

Relate to https://github.com/mosdef-hub/gmso/issues/344. this PR update all python files with py36+ syntax using pyupgrade

PR Checklist

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• [ ] Includes appropriate unit test(s)
• [ ] Appropriate docstring(s) are added/updated
• [ ] Code is (approximately) PEP8 compliant
• [ ] Issue(s) raised/addressed?

[daico007]

Just realize this may not be the best time to do this since it may cause too much conflicts with others' activities