Test python system executable before nodeprep use

[emattiza]

Because of some operating systems choice to deprecate a named system executable of just python, this will test for a suitable python3 if available, otherwise revert to the prior standard command

Issues: #363

Pull Request Checklist

• [x] Review the contributing guidelines
• [x] Outstanding issue linked, if applicable
• [x] PR should only be opened against master if it includes no features (open against develop otherwise)
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Description

Amends the python call to point to python3 if available, else defaults to python. Eliminates command not found error

[alfpark]

Thanks, this was a missed line in using the stored python invocation and will be fixed.