Refactor/core group

[akaviaLab]

• description of feature/fix
  updated group.py and test_group.py to Python 3.6+ compliant code.

[Midnighter]

As always, thank you for your contributions to cobrapy. I'm a bit overwhelmed with all this activity 🙂 It's way more than what I usually spend on cobrapy. @synchon can you maybe review these code updates? You've done a lot of them yourself and I would really appreciate the help.

[synchon]

@Midnighter yeah sure, I can review the updates.

[synchon]

Just a few comments before it's ready. Also, would be nice if you could please run tox -e flake8 to fix the flake8 complaints.

[synchon]

Looks like the PR is not rebased on the latest devel. Can you please rebase it on the latest devel?

[codecov-commenter]

Codecov Report

Merging #1173 (ee22104) into devel (0b3f792) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            devel    #1173      +/-   ##
==========================================
- Coverage   84.06%   84.06%   -0.01%     
==========================================
  Files          65       65              
  Lines        5353     5352       -1     
  Branches     1236     1236              
==========================================
- Hits         4500     4499       -1     
  Misses        545      545              
  Partials      308      308              

Impacted Files	Coverage Δ
src/cobra/core/group.py	89.18% <100.00%> (-0.29%)	⬇️

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[akaviaLab]

Should be rebased now. The commits appear four times because that's what rebasing does on my computer. Gah.

[synchon]

Looks good. Thank you for the contribution!