change % and format to f-strings

[eyllanesc]

Change % and format to f-strings to meet pylint requirements. See #136.

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[codecov]

Codecov Report

Merging #148 (8c1ed7c) into master (16e6ee4) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #148   +/-   ##
=======================================
  Coverage   92.69%   92.69%           
=======================================
  Files           2        2           
  Lines         219      219           
=======================================
  Hits          203      203           
  Misses         16       16           

Flag	Coverage Δ
pytests	92.69% <100.00%> (ø)

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Impacted Files	Coverage Δ
sphinx_tabs/tabs.py	92.62% <100.00%> (ø)

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