StackOverflow for promote_type(Any, Variable)

```julia
julia> promote_type(Any, Variable)
ERROR: StackOverflowError:
Stacktrace:
 [1] term_type(v::Type{Variable})
   @ MultivariatePolynomials ~/.julia/packages/MultivariatePolynomials/iTfZE/src/term.jl:49
 [2] monomial_type(::Type{Variable})
   @ MultivariatePolynomials ~/.julia/packages/MultivariatePolynomials/iTfZE/src/monomial.jl:12
```