Update deprecated methods

[henry2004y]

I noticed the following warning while using PyPlot:

┌ Warning: `haskey(o::PyObject, s::Union{Symbol, AbstractString})` is deprecated, use `hasproperty(o, s)` instead.
│   caller = init_colormaps() at colormaps.jl:57
└ @ PyPlot C:\Users\hyzho\.julia\packages\PyPlot\OOWfD\src\colormaps.jl:57

This PR updates the methods accordingly.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 23.83%. Comparing base (cb34a10) to head (5bbd31d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #590   +/-   ##
=======================================
  Coverage   23.83%   23.83%           
=======================================
  Files           4        4           
  Lines         365      365           
=======================================
  Hits           87       87           
  Misses        278      278           

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[stevengj]

Whoops, thanks