Doc: Add missing python3-dev requirement

[dweindl]

For Debian/Ubuntu for now. Not sure how the package is called on the other distributions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.00%. Comparing base (c3bbdd9) to head (81d14b5).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2584      +/-   ##
===========================================
+ Coverage    77.83%   78.00%   +0.17%     
===========================================
  Files          325      325              
  Lines        21852    21852              
  Branches      1456     1456              
===========================================
+ Hits         17008    17046      +38     
+ Misses        4833     4795      -38     
  Partials        11       11              

Flag	Coverage Δ
cpp	76.46% <ø> (+2.71%)	⬆️
cpp_python	34.21% <ø> (ø)
petab	37.16% <ø> (ø)
python	72.72% <ø> (+0.18%)	⬆️

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see 3 files with indirect coverage changes

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