Update setup.cfg with maintainer info

Author: pepoluan

setup.cfg

@@ -9,6 +9,8 @@ project_urls =
     Bug Tracker = https://github.com/aio-libs/aiosmtpd/issues
     Documentation = https://aiosmtpd.readthedocs.io/
     Source Code = https://github.com/aio-libs/aiosmtpd
+maintainer = The aiosmtpd Developers
+maintainer_email = aiosmtpd@googlegroups.com
 keywords = email, smtpd, smtp
 license = Apache-2.0
 classifiers =