PIDStatus comparator is buggy

[krzysztof]

since __neq__ is not overloaded, the following un-intuitive behaviour happens:

from invenio_pidstore.models import PIDStatus
PIDStatus.NEW == 'N'  # Returns True (expected)
PIDStatus.NEW == 'K'  # Returns False (expected)
PIDStatus.NEW != 'N'  # Returns True (bug)

Not sure if we should overload __neq__ or simply remove the equality comparator and always compare objects instead of their values, because as expected: PIDStatus.NEW != PIDStatus.NEW