StackOverflowError for `isapprox(::Variable, ::Variable)`

```julia
using DynamicPolynomials
@polyvar x

# Passes
@assert 1.2x ≈ 1.2x

# StackOverflowError
x ≈ x
```

The infinite recursion seems to be triggered by:
```julia
promote(x, x) == (x, x)
```