use symbols as `Real` type

I want to use your symbols with the Distributions package, but I get an undefined method error for the symbolic inputs because most distribution parameters are expected to be `Real` while `Sym <: Number`.

example:

```
using SymPy, Distributions
μ, σ = symbols("μ, σ", real=true)
LogNormal(μ, σ)
```

Is it possible to create a symbol that is a `Real` or more general, another type?

Thanks.

PS: it seems all builds and tests are failing, all the buttons are red on github..