fix bugs with pyexists and qt path

[jjstickel]

On my system (MacOS with macports-installed julia and python), PythonPlot was not picking up an appropriate gui backend. It took some sleuthing to determine why, but I discovered that pyimport("importlib") would sometimes not include util as an attribute. This PR fixes that problem, and also adds output of the error in the gui-detection try block to help with any future bugs that might arise there.

I also include in this PR a change to comment out fixqtpath() as it does not seem to work properly, as also reported in #17. I have not spent time chasing this one down. This change enables Qt5 backend on my system, even without specifying ENV["QT_PLUGIN_PATH"].

[codecov-commenter]

Codecov Report

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Additional details and impacted files

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[ufechner7]

Any chance to get this merged, or what is missing?

[jjstickel]

Bump. I was hoping to share code with colleagues that uses PythonPlot, and it would be easier if this is merged.