fix PythonPlot backend

[t-bltg]

Fix #4906, fix #4905, fix #4841.

• restore julia LTS 1.6 compatibility (should not have disappeared !);
• update dependencies versions;
• test julia 1.11.0-alpha2;
• fix CI.

Supersedes #4864.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.67%. Comparing base (fa65e7d) to head (0236bac).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4911      +/-   ##
==========================================
- Coverage   89.77%   89.67%   -0.11%     
==========================================
  Files          40       40              
  Lines        8780     8791      +11     
==========================================
+ Hits         7882     7883       +1     
- Misses        898      908      +10     

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