Deprecate set.is_real property, use set.is_subset(Reals) instead

fixes #6212

(on top of https://github.com/sympy/sympy/pull/7861, see also https://github.com/sympy/sympy/pull/7972)
Is there any way to make XKet and Xbra be real and bounded? Otherwise this result could be Piecewise((x*DiracDelta(x - x_3)*DiracDelta(x_1 - y), Ne(x, -oo)), (S.NaN, True)).