Update qtplaskin.modeldata to be more pythonic3 and fix minor visual bug

[pag1pag]

• Add a try import pyqt everywhere import pyqt5 was used, in order to be compatible with both conda install pyqt and pip install pyqt5
• in qtplaskin.modeldata, use python3 inheritance (super().__init__()) and use Abstractclass for class ModelData
• in qtplaskin.timeformatter, fix a minor visual bug (Instead of xxx s was displayed xxx x_label)

[erwanp]

Build error seems unrelated, I restarted it https://app.travis-ci.com/github/erwanp/qtplaskin/builds/245645423#L662

[codecov[bot]]

Codecov Report

Merging #27 (b297d8a) into master (b06a835) will increase coverage by 1.48%. The diff coverage is 30.13%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
+ Coverage   49.45%   50.94%   +1.48%     
==========================================
  Files          13       13              
  Lines         550      585      +35     
==========================================
+ Hits          272      298      +26     
- Misses        278      287       +9     

Impacted Files	Coverage Δ
qtplaskin/timeformatter.py	29.41% <0.00%> (ø)
setup.py	0.00% <0.00%> (ø)
test/run_test.py	75.00% <ø> (ø)
qtplaskin/modeldata.py	41.84% <32.35%> (+2.13%)	:arrow_up:
test/test_02.py	100.00% <0.00%> (ø)
test/test_01.py	97.56% <0.00%> (+0.06%)	:arrow_up:

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