Install petab on rtd

[kilianvolmer]

I try to resolve #675 by including petab to the dependencies that readthedocs needs to install.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.42%. Comparing base (57099cb) to head (fdbf505).
⚠️ Report is 1 commits behind head on develop.
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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #676      +/-   ##
===========================================
- Coverage    79.48%   79.42%   -0.06%     
===========================================
  Files          142      142              
  Lines         8415     8415              
  Branches      1122     1122              
===========================================
- Hits          6689     6684       -5     
- Misses        1357     1360       +3     
- Partials       369      371       +2     

Flag	Coverage Δ
base	59.33% <ø> (-0.06%)	⬇️
external-R	36.88% <ø> (ø)
external-other-simulators	31.50% <ø> (ø)
migrate	27.10% <ø> (ø)
petab	35.57% <ø> (ø)
visualization	52.59% <ø> (-0.15%)	⬇️

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