Hi all, trying to use `ForwardDiff`

on a Python function and getting errors. Here is a MWE

```
using PyCall
np = pyimport("numpy")
function npsin(x)
np.sin(x)
end
function dnpsin(x)
ForwardDiff.derivative(npsin, x)
end
```

`dnpsin(π/2)`

gives following error

```
MethodError: no method matching Float64(::ForwardDiff.Dual{ForwardDiff.Tag{typeof(npsin),Float64},Float64,1})
Closest candidates are:
Float64(::Real, ::RoundingMode) where T<:AbstractFloat at rounding.jl:200
Float64(::T) where T<:Number at boot.jl:716
Float64(::Irrational{:mad_constant}) at irrationals.jl:189
...
Stacktrace:
[1] convert(::Type{Float64}, ::ForwardDiff.Dual{ForwardDiff.Tag{typeof(npsin),Float64},Float64,1}) at ./number.jl:7
[2] cconvert(::Type{T} where T, ::ForwardDiff.Dual{ForwardDiff.Tag{typeof(npsin),Float64},Float64,1}) at ./essentials.jl:388
[3] macro expansion at /home/arif/.julia/packages/PyCall/tqyST/src/exception.jl:95 [inlined]
[4] PyObject(::ForwardDiff.Dual{ForwardDiff.Tag{typeof(npsin),Float64},Float64,1}) at /home/arif/.julia/packages/PyCall/tqyST/src/conversions.jl:23
[5] _pycall!(::PyObject, ::PyObject, ::Tuple{ForwardDiff.Dual{ForwardDiff.Tag{typeof(npsin),Float64},Float64,1}}, ::Int64, ::Ptr{Nothing}) at /home/arif/.julia/packages/PyCall/tqyST/src/pyfncall.jl:24
[6] _pycall! at /home/arif/.julia/packages/PyCall/tqyST/src/pyfncall.jl:11 [inlined]
[7] #_#116 at /home/arif/.julia/packages/PyCall/tqyST/src/pyfncall.jl:86 [inlined]
[8] (::PyObject)(::ForwardDiff.Dual{ForwardDiff.Tag{typeof(npsin),Float64},Float64,1}) at /home/arif/.julia/packages/PyCall/tqyST/src/pyfncall.jl:86
[9] npsin(::ForwardDiff.Dual{ForwardDiff.Tag{typeof(npsin),Float64},Float64,1}) at ./In[181]:2
[10] derivative at /home/arif/.julia/packages/ForwardDiff/sqhTO/src/derivative.jl:14 [inlined]
[11] dnpsin(::Float64) at ./In[185]:2
[12] top-level scope at In[187]:1
[13] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1091
```