store prefs in depot_path/prefs (#589)
store prefs in depot_path/prefs (#589)

* store prefs in depot_path/prefs

As a stopgap until we get proper package options (JuliaLang/Juleps#38), store PYTHON preference in depot_path/prefs/PyCall so that they aren't forgotten when PyCall is updated

* whoops

JuliaPy/PyCall.jl