Fix reading problem.x_names from HDF5

[dweindl]

From h5py 3.0.0 on, strings are read as bytes instrad of str, so we need to convert.

[codecov-io]

Codecov Report

Merging #528 (e50ae7f) into develop (c807d4b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #528      +/-   ##
===========================================
+ Coverage    90.69%   90.71%   +0.02%     
===========================================
  Files           65       65              
  Lines         4212     4212              
===========================================
+ Hits          3820     3821       +1     
+ Misses         392      391       -1     

Impacted Files	Coverage Δ
pypesto/store/read_from_hdf5.py	95.34% <100.00%> (ø)
pypesto/optimize/optimizer.py	90.83% <0.00%> (+0.26%)	⬆️

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[dweindl]

Hm, what's going on with test_unbounded_minimize here? Just plenty of warnings

test_optimization[integrated-optimizer39] seems to fail irregularly / frequently.