fix the error in debugging when pdb.Pdb() has .run instead of ._runscript method

[xingnix]

PR Summary

When debugging with elpy, there is "no _runscript error" with new version of Python where Python's debug module pdb removed _runscript method from object created by pdb.Pdb().
The changed codes use pdb.Pdb().run when there is no _runscript to do debuging.

PR checklist

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• Commits respect our guidelines
• Tests are passing properly (see here on how to run Elpy's tests)