Drop usage of unsupported pip api

[kvikshaug]

From pip v9.0.2, importing pip after certain packages, like requests, fails. That can cause problems with applications using cobrapy. Additionally, pip now makes it pretty clear that the internal api is not supported.

This PR removes package print output from show_versions and instead encourages issue reporters to include the output of pip list.

[cdiener]

Hi thanks that will be changed. I think we will still want to stick with having show_versions since using pip has some pitfalls (pip might not belong to the same distribution as the notebook, python2 and python3 installed at the same time, Windows users sometimes are confused where to run that command, etc). However, we are in the process of switching to pkg_resources so it won't use the pip API anymore. Please see #697 as well.

Thanks and sorry for the delay!

[Midnighter]

I agree, #697 is the better solution to the problem. Thank you for spotting this so early, though. We should have been warned :/

[codecov-io]

Codecov Report

Merging #680 into devel will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            devel     #680      +/-   ##
==========================================
- Coverage   81.77%   81.72%   -0.06%     
==========================================
  Files          36       36              
  Lines        3644     3633      -11     
  Branches      832      829       -3     
==========================================
- Hits         2980     2969      -11     
  Misses        470      470              
  Partials      194      194

Impacted Files	Coverage Δ
cobra/util/version_info.py	100% <ø> (ø)	⬆️

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