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 <utzig@apache.org>

scripts/setup.py

@@ -5,7 +5,7 @@ setuptools.setup(
     name="imgtool",
     version=imgtool_version,
     author="The MCUboot commiters",
-    author_email="None",
+    author_email="dev-mcuboot@lists.runtime.co",
     description=("MCUboot's image signing and key management"),
     license="Apache Software License",
     url="http://github.com/JuulLabs-OSS/mcuboot",