pywikiapi: why + update

[benoit74]

We should:

• document why we are using pywikiapi instead of pywikibot (there is probably a good reason but it seems it is about to get lost)
• use upstream pywikiapi package (PyPi) instead of custom repo, now that our changes have been merged and we have write access to the upstream repo: Enhancements from openZIM devs nyurik/pywikiapi#3

[rgaudin]

Git commit introducing it doesn't mention this.

I don't recall which options were considered and what were the reasons but I'd guess it's because our usage of the API is very limited and because pywikiapi is just a thin wrapper handling pagination in our case.

Feel free to reconsider.

[benoit74]

@kelson42 mentioned on slack we might want to consider Pywikibot instead of Pywikiapi