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? 

I don't know what you mean by this.  But if you do care about this test - we can preserve it, see https://github.com/sympy/sympy/pull/7972

But lets discuss https://github.com/sympy/sympy/pull/7972 and https://github.com/sympy/sympy/pull/7861 not in this pr.