Use `completing-read-multiple` for multiple files selection

[meliache]

I would prefer if the package used completing-read-multiple in python-pytest--select-test-files rather than just completing-read in a while loop with an extra candidate to finish completion. In python-pytest--select-test-files, the final part of the function body could be simplified to (completing-read-multiple "Choose test files: " candidates). The user can then just use normal interactive completion to select a file and if they want to select multiple files, they just enter a comma or whatever their value of crm-separator is. The advantage is that it requires less user inputs if the user just wants to select a single file or diretory and you don't need to rely on the user finding the special "finish completion" candidate.

[wbolster]

yeah, makes total sense, guess i wasn't aware of that when i built the convoluted logic before. fixed via f945da6