Fix package build sources; add pyproject toml for build deps

[yannikschaelte]

previously, automatic package detection included the test/ module.

[codecov-io]

Codecov Report

Merging #634 (6e92bd4) into develop (160c2a8) will decrease coverage by 53.03%.
The diff coverage is 24.32%.

@@             Coverage Diff              @@
##           develop     #634       +/-   ##
============================================
- Coverage    88.16%   35.13%   -53.04%     
============================================
  Files           79       93       +14     
  Lines         5257     5898      +641     
============================================
- Hits          4635     2072     -2563     
- Misses         622     3826     +3204     

Impacted Files	Coverage Δ
pypesto/ensemble/covariance_analysis.py	18.36% <0.00%> (-0.39%)	⬇️
pypesto/objective/aesara.py	0.00% <0.00%> (ø)
pypesto/objective/aggregated.py	25.49% <0.00%> (-74.51%)	⬇️
pypesto/objective/amici_calculator.py	25.00% <0.00%> (-66.67%)	⬇️
pypesto/petab/__init__.py	0.00% <0.00%> (ø)
pypesto/petab/importer.py	0.00% <0.00%> (-85.90%)	⬇️
pypesto/petab/pysb_importer.py	0.00% <0.00%> (-100.00%)	⬇️
pypesto/visualize/parameters.py	14.14% <0.00%> (-77.78%)	⬇️
pypesto/visualize/waterfall.py	14.81% <0.00%> (-81.49%)	⬇️
pypesto/store/read_from_hdf5.py	50.00% <4.54%> (-7.90%)	⬇️
... and 102 more

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