utils: arXiv identifier normalization

• Adds support for normalizing arxiv identifiers according to http://arxiv.org/help/arxiv_identifier and http://arxiv.org/help/arxiv_identifier_for_services. (closes #1958, #1961)

• Fixes issues with pre-2007 arxiv identifier validation.

• Fixes PEP257 problems.

• Adds tests including missing assertion of to_url() method.

Co-authored-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>