DEP: Deprecate that comparisons ignore errors.

This means that for example broadcasting errors get raised.

The array_equiv function is changed to explicitely test
if broadcasting is possible. It may be nice to do this
test differently, but I am not sure if that is possible.

Create a FutureWarning for comparisons to None, which
should result in areal elementwise (object) comparisons.

Slightly adepted a wrong test.

Poly changes: Some changes in the polycode was necessary,
the one is probably a bug fix, the other needs to be
thought over, since len check is not perfect maybe, since
it is more liekly to raise raise an error.

Closes gh-3759 and gh-1608
DOC: Add (object) equality changes and error raising to release notes
DEP: Deprecate identity check in comparisons (not just removing)

Instead of just removing it, raise Deprecation/Future warnings
when the comparison object ufunc and the identity check do not
coincide.
MAINT: Comparison deprecation followup fixes

Makes the identity check `a = np.array([np.nan], dtype=object)`
`a == a`, etc. a deprecation/futurewarning instead of just
changing it.
Also fixes some smaller things.