Update to libzim 8

[rgaudin]

Uses libzim8 release and updates tests & release worflow:

• macos-10.15 has been removed
• macos-12 has no default python3.6 anymore. Using python-versions's 3.6.15 (downloaded)
• pytest7 now requires pythonpath in pyproject.toml
• using latest minor cython 0.29.32
• Updating blank ZIM's expected size. libzim8 creates a slightly larger ZIM for this.

[codecov[bot]]

Codecov Report

Merging #152 (ef1e377) into master (4140999) will not change coverage. The diff coverage is n/a.

:exclamation: Current head ef1e377 differs from pull request most recent head 6eff603. Consider uploading reports for the commit 6eff603 to get more accurate results

@@           Coverage Diff           @@
##           master     #152   +/-   ##
=======================================
  Coverage   96.88%   96.88%           
=======================================
  Files           1        1           
  Lines         417      417           
=======================================
  Hits          404      404           
  Misses         13       13           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[rgaudin]

Sorry about the noise ; had difficulties finding what made the release workflow randomly fail.