MFH: r509276 net-im/py-slackclient: Update to 2.1.0

aiodns is (was) actually an optional dependency. Add an ASYNCDNS option
for now until upstream work out a good name for it [1]

Changelog:

https://slack.dev/python-slackclient/changelog.html#v2-1-0-2019-07-01

Note: Missing from changelog, but also fixed:

https://github.com/slackapi/python-slackclient/issues/451

[1] https://github.com/slackapi/python-slackclient/issues/433

PR: 239462
Submitted by: <wcarson.bugzilla disillusion.net>
Approved by: ports-secteam (blanket: bugfix release)