Fix #2534 and #2535 Test in wikipedia and arxiv

[fnielsen]

Doctest in arxiv.py failed because of change in error message from the arxiv server. A more general test is now done in the doctest which is not dependent on the specicific error message.

A doctest in wikipedia.py failed because the default requests user agent was banned by the Wikidata Query Service. Now the user agent is set based on the Scholia configuration.

Fixes #2534 and #2535

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