binman: fallback to importlib_resources on Python 3.8

binman: fallback to importlib_resources on Python 3.8

Python 3.7 and 3.8 lack the files attribute in importlib.resources.
Use importlib_resources to fix build errors with Python 3.8:
binman: module 'importlib.resources' has no attribute 'files'

Fixes: 538719cb6a77 ("binman: migrate from pkg_resources to importlib")
Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>
Acked-by: Quentin Schulz <quentin.schulz@cherry.de>
[trini: Re-add # pragma: no cover line]
Signed-off-by: Tom Rini <trini@konsulko.com>