Fix #2534 and #2535 Test in wikipedia and arxiv

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.

scholia/arxiv.py

@@ -66,8 +66,8 @@ def get_metadata(arxiv):
     >>> metadata = get_metadata('1503.00759')
     >>> metadata['doi'] == '10.1109/JPROC.2015.2483592'
     True
-    >>> get_metadata('5432.01234')
-    {'error': 'Not found'}
+    >>> 'error' in get_metadata('5432.01234')
+    True
 
     """
     arxiv = arxiv.strip()

scholia/wikipedia.py

@@ -110,7 +110,8 @@ def q_to_bibliography_templates(q):
     query = BIBLIOGRAPHY_SPARQL_QUERY.format(q=q)
     url = SPARQL_ENDPOINT
     params = {'query': query, 'format': 'json'}
-    response = requests.get(url, params=params)
+    headers = {'User-Agent': config['requests'].get('user_agent')}
+    response = requests.get(url, params=params, headers=headers)
     data = response.json()
 
     wikitext = ('<!-- Generated with scholia.wikipedia '