sanitylib: pacify pylint

According to our compliance checks, we have 'useless-suppression'
and 'unused-import' on our hands:

W0611: Unused CLoader imported from yaml as Loader
W0611: Unused Loader imported from yaml
I0021: Useless suppression of 'unsubscriptable-object'

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>