Manually install ipopt as URLs are not allowed in install_requires on pypi

[yannikschaelte]

see e.g. pypa/pip#6301

0.2.4 was manually uploaded to pypi after automatic deploy failed

[codecov-io]

Codecov Report

Merging #599 (9920547) into develop (2f31e0e) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #599      +/-   ##
===========================================
- Coverage    89.60%   89.56%   -0.04%     
===========================================
  Files           91       91              
  Lines         5732     5732              
===========================================
- Hits          5136     5134       -2     
- Misses         596      598       +2     

Impacted Files	Coverage Δ
pypesto/sample/geweke_test.py	91.54% <0.00%> (-2.82%)	⬇️

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