metadata: review and lint code

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[codecov]

Codecov Report

Attention: Patch coverage is 99.00990% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.63%. Comparing base (948c47f) to head (1337f41).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
trafilatura/metadata.py	99.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #694      +/-   ##
==========================================
+ Coverage   98.55%   98.63%   +0.08%     
==========================================
  Files          21       21              
  Lines        3536     3525      -11     
==========================================
- Hits         3485     3477       -8     
+ Misses         51       48       -3     

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