Add completion tests for several libraries of the Scientific Python stack

[ccordoba12]

This will allow us to verify that Jedi doesn't break completions for libraries like Numpy, Pandas and Matplotlib (the ones tested here).

[ccordoba12]

@gatesn, this clearly shows that Jedi 0.15 broke completions for those libraries, which we discovered when we tried to update our Spyder dependency on the python-language-server to use version 0.29.

Therefore, I'm considering to push a new PR that adds back support for Jedi 0.14.1, along with 0.15, to have a python-language-server that works for Numpy, Pandas, etc. And on the Spyder side we'd demand 0.14.1 to have the version that we really need.

This is my plan because I think we can't demand Jedi <0.15 here again due to 0.29+ being already packaged by several Linux distros (e.g. Arch).

Please let me know if you think there's a better way to handle this very ugly situation. If I don't hear from you by tomorrow, I'll continue with this plan because it's critical for us to start using 0.29+ due to the PRs submitted by our team (#680, #665, #655 and more to come).