[REF] fix f strings

[Remi-Gau]

Relates to #2528

Changes proposed in this pull request:

• format remaining f strings in
  • examples
  • maskers
  • reporting
  • utils

[github-actions]

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[codecov]

Codecov Report

Merging #3770 (dbac40f) into main (6a576e5) will increase coverage by 0.00%.
The diff coverage is 95.23%.

@@           Coverage Diff           @@
##             main    #3770   +/-   ##
=======================================
  Coverage   91.65%   91.65%           
=======================================
  Files         139      139           
  Lines       16554    16562    +8     
  Branches     3230     3229    -1     
=======================================
+ Hits        15172    15180    +8     
  Misses       1379     1379           
  Partials        3        3           

Impacted Files	Coverage Δ
nilearn/_utils/helpers.py	78.26% <0.00%> (ø)
nilearn/_utils/param_validation.py	100.00% <ø> (ø)
nilearn/_utils/testing.py	68.68% <ø> (ø)
nilearn/datasets/func.py	86.03% <ø> (ø)
nilearn/interfaces/bids/_utils.py	100.00% <ø> (ø)
..._testing/_glm_reporter_visual_inspection_suite_.py	88.23% <ø> (ø)
nilearn/_utils/data_gen.py	98.43% <100.00%> (ø)
nilearn/_utils/extmath.py	94.73% <100.00%> (ø)
nilearn/_utils/niimg.py	99.01% <100.00%> (ø)
nilearn/maskers/nifti_labels_masker.py	93.78% <100.00%> (+0.14%)	⬆️
... and 1 more

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[Remi-Gau]

There should be one more PR for f strings after that and then nilearn should be fully f stringified

[bthirion]

LGTM, thx !