Fix import of pybliometrics exceptions

[larsgrobe]

Dear all, with current versions of pybliometrics, the import of ScopusXXXErrors has to refer to the module pybliometrics.exception, not pybliometrics.scopus.exception. This affects line 48 in sources/scopus.py.

Best, Lars.

[stijnh]

Thanks! Could you specifiy which version of pybliometrics you're using? It has changed between versions, so we may need to fix the version in requirements.txt