codespell_lib/_codespell.py: read additional args from file with with…

[julian-smith-artifex-com]

This allows arguments to be read from a file with @PATH.

It uses https://docs.python.org/3/library/argparse.html#fromfile-prefix-chars.

I'd like this change specifically to avoid having to specify a large number of paths on the command line - when codespell reports errors, ptest dumps the entire command line which makes it difficult to find the actual codespell output.

There is currently one test failure, of codecov/patch. I don't understand what's going on here - the new function convert_arg_line_to_args() is definitely called by new test test_args_from_file().

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[julian-smith-artifex-com]

I'm not sure why the codecov/patch test is failing - the new test definitely ends up using the new convert_arg_line_to_args() function.