Update to 0.7- iteration protocol (start/next/done -> iterate)

[schmrlng]

I've spent the past week hoping that someone better versed in the new iteration protocol than me might take a crack at this, but at least the tests pass.

The remaining deprecation warnings in pkg> test PyCall (see below) are mostly related to implicit conversion/construction of Number types; it's not quite the same issue but I'll wait for the resolution of JuliaDiff/ForwardDiff.jl#342 before defining

(::Type{T})(x::PyObject) where {T<:Number} = convert(T, x)

or at the very least,

(::Type{T})(x::PyObject) where {T<:Integer} = convert(T, x)

---

┌ Info: Python version 3.6.5 from libpython3.6m, PYTHONHOME=/usr:/usr
│ ENV[PYTHONPATH]=/opt/ros/melodic/lib/python2.7/dist-packages:/home/schmrlng/catkin_ws/devel/lib/python2.7/dist-packages
│ ENV[PYTHONHOME]=
└ ENV[PYTHONEXECUTABLE]=
┌ Warning: Constructors no longer fall back to `convert`. A constructor `BigInt(::PyObject)` should be defined instead.
│   caller = top-level scope at runtests.jl:237
└ @ Core ~/.julia/dev/PyCall/test/runtests.jl:237
┌ Warning: Constructors no longer fall back to `convert`. A constructor `Int64(::PyObject)` should be defined instead.
│   caller = top-level scope at runtests.jl:242
└ @ Core ~/.julia/dev/PyCall/test/runtests.jl:242
┌ Warning: InexactError now supports arguments, use `InexactError(funcname::Symbol, ::Type, value)` instead.
│   caller = ip:0x0
└ @ Core :-1
┌ Warning: Constructors no longer fall back to `convert`. A constructor `Int64(::PyObject)` should be defined instead.
│   caller = top-level scope at runtests.jl:308
└ @ Core ~/.julia/dev/PyCall/test/runtests.jl:308
┌ Warning: Constructors no longer fall back to `convert`. A constructor `Int64(::PyObject)` should be defined instead.
│   caller = top-level scope at runtests.jl:308
└ @ Core ~/.julia/dev/PyCall/test/runtests.jl:308
Test Summary: | Pass  Total
PyCall        |  412    412
Test Summary: | Pass  Total
pydef         |    6      6
Test Summary: | Pass  Total
pycall!       |   16     16
   Testing PyCall tests passed

[mkborregaard]

Would be nice to tag a release with this.