f-string guidelines using Flynt

[SubhadityaMukherjee]

Reference Issue

Fixes #1295

What does this PR implement/fix? Explain your changes.

As Eddie and Pieter mentioned in #1295, this runs Flynt (a compliancy checker for f-strings) on the codebase, and adds a guideline for future contributors to ensure compliance.

How should this PR be tested?

This does not add any new functionality. Just converts non f-strings to f-strings, so hopefully all the current tests should just work.

Any other comments?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.76923% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.09%. Comparing base (483f467) to head (2cb58a0).

Files with missing lines	Patch %	Lines
openml/setups/setup.py	0.00%	3 Missing ⚠️
openml/evaluations/evaluation.py	0.00%	1 Missing ⚠️
openml/extensions/sklearn/extension.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1406      +/-   ##
===========================================
+ Coverage    84.05%   84.09%   +0.04%     
===========================================
  Files           38       38              
  Lines         5230     5231       +1     
===========================================
+ Hits          4396     4399       +3     
+ Misses         834      832       -2     

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[PGijsbers]

Part of the old text also became obsolete.

[PGijsbers]

✏️