New round and API-only export test

[Federico Leva (Nemo)]

I've finally committed a test of API-only export which won't use
Special:Export any more:
<https://github.com/WikiTeam/wikiteam/blob/be5ca12075c8de3a7d3d297ecff21fd85c661417/dumpgenerator.py>

Can you please test it and see if it exports all the revisions that it
should? The first wiki I exported this way is:
https://archive.org/details/wiki-wikiopennmsorg_wiki

(It didn't work with the usual method.)

I had hoped to use generators, but I'm not sure how to tell the API I
want all revisions for a page, and I didn't get all of them:
<https://github.com/WikiTeam/wikiteam/commit/a8cbb357ff859ace128f342d92d148938124e4b4>

Sorry for the git mess. It was supposed to be a quick fix to complete my
current round of updates of ~10k alive wikis from the wikiteam
collection. I "only" downloaded ~6k so far in the last ~3 days. I'll
soon launch this version over the remaining wikis.

Federico