Turn method importing.ImportDialog._unicodeWarning() into a function.

Hi. This is one of a few problems i have spotted with pylint. This one is the only one were the fix is obvious and easy.

It looks like `importing.ImportDialog.importFile()` was turned into a function `importing.importFile()` at some time.

But then `importing.ImportDialog._unicodeWarning()`, which is called from there, must be turned into a function as well.