add is_live test using HTTP HEAD request

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Codecov Report

Merging #327 (acf2ec0) into master (21d793d) will decrease coverage by 0.29%.
The diff coverage is 95.45%.

❗ Current head acf2ec0 differs from pull request most recent head b0cc60d. Consider uploading reports for the commit b0cc60d to get more accurate results

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@@            Coverage Diff             @@
##           master     #327      +/-   ##
==========================================
- Coverage   96.49%   96.20%   -0.29%     
==========================================
  Files          22       22              
  Lines        3336     3377      +41     
==========================================
+ Hits         3219     3249      +30     
- Misses        117      128      +11     

Impacted Files	Coverage Δ
trafilatura/downloads.py	97.02% <86.20%> (-2.29%)	⬇️
trafilatura/sitemaps.py	95.62% <98.57%> (-0.15%)	⬇️
trafilatura/cli.py	94.70% <100.00%> (-0.04%)	⬇️
trafilatura/cli_utils.py	91.03% <100.00%> (-0.48%)	⬇️

... and 2 files with indirect coverage changes

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