Add tests that exercise the version pins in setup.py

[mpenkov]

I personally feel it's OK (and even beneficial) for fresh gensim releases to require fairly-recent releases of those libraries

Yes, upgrading numpy is not a problem. I'd expect our tests to fail though. I'm pretty sure we were testing against the oldest lib, explicitly, but with all the recent changes to the testing framework (azure etc) it's possible that fell through the cracks. We should put it back if possible, @mpenkov WDYT?

Originally posted by @piskvorky in #2899 (comment)