move _current_plots_version from PlotDocs to Plots

src/Plots.jl

@@ -1,5 +1,7 @@
 module Plots
 
+_current_plots_version = v"0.20.1"
+
 using Reexport
 
 import StaticArrays

test/imgcomp.jl

@@ -29,7 +29,7 @@ default(size=(500,300))
 # TODO: use julia's Condition type and the wait() and notify() functions to initialize a Window, then wait() on a condition that
 #       is referenced in a button press callback (the button clicked callback will call notify() on that condition)
 
-const _current_plots_version = v"0.20.1"
+import Plots._current_plots_version
 
 
 function image_comparison_tests(pkg::Symbol, idx::Int; debug = false, popup = isinteractive(), sigma = [1,1], tol = 1e-2)