chore: use f-strings where applicable

[nijel]

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[codecov]

Codecov Report

Attention: Patch coverage is 26.66667% with 11 lines in your changes missing coverage. Please review.

Project coverage is 77.91%. Comparing base (326d4b3) to head (2aa1274).
Report is 33 commits behind head on master.

Files with missing lines	Patch %	Lines
social_core/backends/discord.py	0.00%	5 Missing ⚠️
social_core/backends/clever.py	0.00%	2 Missing ⚠️
social_core/backends/classlink.py	0.00%	1 Missing ⚠️
social_core/backends/fitbit.py	0.00%	1 Missing ⚠️
social_core/backends/pocket.py	0.00%	1 Missing ⚠️
social_core/backends/shimmering.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #979      +/-   ##
==========================================
+ Coverage   77.88%   77.91%   +0.02%     
==========================================
  Files         347      347              
  Lines       10669    10664       -5     
  Branches      504      451      -53     
==========================================
- Hits         8310     8309       -1     
+ Misses       2200     2199       -1     
+ Partials      159      156       -3     

Flag	Coverage Δ
unittests	77.91% <26.66%> (+0.02%)	⬆️

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