use Requires.jl to make weak dependencies work

Based on
https://pkgdocs.julialang.org/v1/creating-packages/#Backwards-compatibility
this uses Requires.jl to bring the weak dependency feature to Julia <1.9

Project.toml

@@ -11,17 +11,11 @@ Images = "916415d5-f1e6-5110-898d-aaa5f9f070e0"
 Interpolations = "a98d9a8b-a2ab-59e6-89dd-64a1c18fca59"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 Printf = "de0858da-6303-5e67-8744-51eddeeeb8d7"
-PyPlot = "d330b81b-6aea-500a-939a-2ce795aea3ee"
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
+Requires = "ae029012-a4dd-5104-9daa-d747884805df"
 Statistics = "10745b16-79ce-11e8-11f9-7d13ad32a3b2"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
-[weakdeps]
-PyPlot = "d330b81b-6aea-500a-939a-2ce795aea3ee"
-
-[extensions]
-ImageProjectiveGeometryPyPlotExt = "PyPlot"
-
 [compat]
 DSP = "0.5 - 0.7"
 FileIO = "1"
@@ -30,3 +24,12 @@ Images = "0.20 - 0.25"
 Interpolations = "0.10 - 0.14"
 PyPlot = "2.5 - 2.11"
 julia = "1.4"
+
+[extensions]
+ImageProjectiveGeometryPyPlotExt = "PyPlot"
+
+[extras]
+PyPlot = "d330b81b-6aea-500a-939a-2ce795aea3ee"
+
+[weakdeps]
+PyPlot = "d330b81b-6aea-500a-939a-2ce795aea3ee"

ext/ImageProjectiveGeometryPyPlotExt.jl

@@ -1,8 +1,9 @@
 module ImageProjectiveGeometryPyPlotExt
 using LinearAlgebra
-using ImageProjectiveGeometry
 using PyPlot
 
+isdefined(Base, :get_extension) ? (using ImageProjectiveGeometry) : (using ..ImageProjectiveGeometry)
+
 include("ransacdemo.jl")
 
 

src/ImageProjectiveGeometry.jl

@@ -165,11 +165,15 @@ include("ransac.jl")
 include("cornerfeatures.jl")
 include("utilities.jl")
 include("geometry.jl")
-include("plotting.jl")
 
-# to keep compatibility with older Julia Versions, include this if we cannot do weak deps
+# This symbol is only defined on Julia versions that support extensions
 if !isdefined(Base, :get_extension)
-  include("../ext/ImageProjectiveGeometryPyPlotExt.jl")
+using Requires
+end
+if !isdefined(Base, :get_extension)
+function __init__()
+    Requires.@require PyPlot="d330b81b-6aea-500a-939a-2ce795aea3ee" include("../ext/ImageProjectiveGeometryPyPlotExt.jl")
+end
 end
 
 end  # module