utils: fixes to arXiv id regex

* Allows 'ar[xX]iv:' prefix not to be included in the arXiv ID.

Signed-off-by: Pedro Gaudêncio <pmgaudencio@gmail.com>
Reviewed-by: Javier Martin Montull <javier.martin.montull@cern.ch>
`re.I`  is `re.IGNORECASE`, so `x` is the same as `[xX]`