isapprox stackoverflows for monomial/variable expressions

[DanielVandH]

julia> using DynamicPolynomials

julia> @polyvar x
(x,)

julia> x + 1 ≈ x + 1 # works fine
true

julia> typeof(x+1)
Polynomial{DynamicPolynomials.Commutative{DynamicPolynomials.CreationOrder}, Graded{LexOrder}, Int64}

julia> x ≈ x
Warning: detected a stack overflow; program state may be corrupted, so further execution might be unreliable.
ERROR: StackOverflowError:
Stacktrace:
     [1] isapprox(p1::Variable{…}, p2::Variable{…}; kwargs::@Kwargs{})
       @ MultivariatePolynomials C:\Users\djv23\.julia\packages\MultivariatePolynomials\yR88J\src\operators.jl:25
     [2] isapprox(p1::Variable{…}, p2::Variable{…})
       @ MultivariatePolynomials C:\Users\djv23\.julia\packages\MultivariatePolynomials\yR88J\src\operators.jl:24
--- the above 2 lines are repeated 39990 more times ---
 [79983] isapprox(p1::Variable{…}, p2::Variable{…}; kwargs::@Kwargs{})
       @ MultivariatePolynomials C:\Users\djv23\.julia\packages\MultivariatePolynomials\yR88J\src\operators.jl:25
Some type information was truncated. Use `show(err)` to see complete types.

julia> typeof(x)
Variable{DynamicPolynomials.Commutative{DynamicPolynomials.CreationOrder}, Graded{LexOrder}}

julia> x^0 ≈ x^0
Warning: detected a stack overflow; program state may be corrupted, so further execution might be unreliable.
ERROR: StackOverflowError:
Stacktrace:
     [1] isapprox(p1::DynamicPolynomials.Monomial{…}, p2::DynamicPolynomials.Monomial{…})
       @ MultivariatePolynomials C:\Users\djv23\.julia\packages\MultivariatePolynomials\yR88J\src\operators.jl:24
     [2] isapprox(p1::DynamicPolynomials.Monomial{…}, p2::DynamicPolynomials.Monomial{…}; kwargs::@Kwargs{})
       @ MultivariatePolynomials C:\Users\djv23\.julia\packages\MultivariatePolynomials\yR88J\src\operators.jl:25
--- the above 2 lines are repeated 39990 more times ---
 [79983] isapprox(p1::DynamicPolynomials.Monomial{…}, p2::DynamicPolynomials.Monomial{…})
       @ MultivariatePolynomials C:\Users\djv23\.julia\packages\MultivariatePolynomials\yR88J\src\operators.jl:24
Some type information was truncated. Use `show(err)` to see complete types.

julia> typeof(x^0)
DynamicPolynomials.Monomial{DynamicPolynomials.Commutative{DynamicPolynomials.CreationOrder}, Graded{LexOrder}}

[blegat]

That's an oversight, thanks for reporting it