fix missing z for 3d markerplots in PyPlot

[fhagemann]

This is an attempt to fix #3036.

For 3D plots, it seems like the z array is not passed, such that the markers are plotted at z = 0.

using Plots; pyplot()
plot([1,2,3],[1,3,2],[3,1,2], label = "Line")
scatter!([1,2,3],[1,3,2],[3,1,2], label = "Scatter", camera = (0,0), zlims = (0,3))

Old code:

New code:

I am quite new to the Plots.jl code, so I cannot judge whether this pull request completely fixes the issue.

Feedback is very welcome!

[daschw]

Nice thanks a lot!