Added global option to disable 'Python:' prefix in show

[J-C-Q]

For my use case this functionality is nice to have. Maybe others think so too, thus the pull request :)

[hhaensel]

I personally wouldn't need it, but if you propose it, why not distinguishing between Julia versions?

@static if VERSION < v"1.8"
    printPyPrefix = true
else
    printPyPrefix::Bool = true
end