net-im/py-slackclient: Update to 1.2.1

• Python 3 is now supported [1].
• Update and sort all *_DEPENDS to match upstreams declarations, using specific versions where declared and possible (eg: we dont use "!=").
• We want one line per test ouput, so verbosify the tests.

[1] https://github.com/slackapi/python-slackclient/issues/165

Changelog:

https://slackapi.github.io/python-slackclient/changelog.html

PR: 228137
Submitted by: Derek Schrock <dereks lifeofadishwasher com>