Converted all .format() to f-strings.

[PabloLec]

I converted all appearance of .format() to f-string.

• [✓] I have read the contribution guidelines
• [ / ] CI Unit tests pass (Pytest already raised errors for widgets)
• [✓] New functions/classes have consistent docstrings

What this pull request changes

• Conversion to f-string implies that only Python 3.6+ will be supported.

Issues fixed with this pull request

• Issue Only support python 3.6+ going forward + migrate to use some 3.6 features #109. Partially, we need to determine if any other 3.6 features integrations would be relevant.

[jwlodek]

Thanks!