Fix error introduced by changing PyPlot to Plots

6 jobs for ci_tests in 8 minutes and 25 seconds (queued for 1 second)
Name Stage Failure
failed
test:latest Test
    @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:75 [inlined]
[8] #test#62
@ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:74 [inlined]
[9] test(pkg::String)
@ Pkg.API /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Pkg/src/API.jl:74
[10] top-level scope
@ none:1
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1