Allow passing filenames to --mbox that contain parentheses

Allow passing filenames to --mbox that contain parentheses

Without this you would get an error like this:

Failed to parse mbox:
b'/bin/sh: 1: Syntax error: "(" unexpected\n'

This especially matters when loading files downloaded with a browser,
since those often contain (1) or (2) if a file with the same name was
downloaded earlier.