Forbid incompatible IntervalArithmetic v0.21

[schillic]

This is mainly to prevent attempts from CompatHelper.

Since there are explicit tests for Rationals, I presumed that they should be supported. But they were dropped in IntervalArithmetic v0.21. For the record, this is the first error in the tests:

Univariate monomial, rational: Error During Test at BernsteinExpansions/test/runtests.jl:24
  Test threw exception
  Expression: univariate(x ^ 3, 3, Interval(1 // 1, 2 // 1)) == [1 // 1, 2 // 1, 4 // 1, 8 // 1]
  MethodError: no method matching Interval(::Rational{Int64}, ::Rational{Int64})

[lucaferranti]

the following works

julia> interval(1//2, 2//1)
[1//2, 2//1]

[schillic]

Oh I see, the whole constructor Interval was removed. Then I will change here.

[mforets]

ok, this will need quite some changes in downstream packages. we can start addressing that tomorrow..

edit: for a moment i thought IA v1.0 was released and some breaking changes were introduced (such as removing IntervalBox), but its' not the case yet.

[schillic]

There are many breaking changes actually (but IntervalBox is still there ... for now). For instance, the constructor Interval(...) was removed. This causes the error above. But I realized (thanks @lucaferranti) that we can just use the function interval instead. Since this constructor was only used in the tests, this did not even require changes to the code for this package.

I close this in favor of #45.