missing `.copy()` method for `Dict` wrapper

[stevengj]

From JuliaPy/PythonPlot.jl#9

julia> Py([3,4,5]).copy()                          # works
Python VectorValue: <jl [3, 4, 5]>

julia> Py(Dict(1 => 2, 3 => 4)).copy()             # fails
ERROR: Python: AttributeError: Julia: type Dict has no field copy
Python stacktrace:

(Somewhat different from #35.)

[cjdoris]

Good spot, I've added it.

[stevengj]

Thanks!