imgtool: add mailing list email to setup
imgtool: add mailing list email to setup

Seems like pypi.org is not accepting "None" for email anymore, so this
just adds the mailing list in case someone needs to contact the project.

Signed-off-by: Fabio Utzig <66676b1ceaf93296e098708c09494361103aa635@apache.org>

utzig/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,ATmobica/mcuboot,utzig/mcuboot