net-im/py-slackclient: Update to 1.0.0

• Update PORTVERSION and distinfo checksum (1.0.0)
• Switch to GitHub for DISTFILES (includes tests) [1]
• Add test dependencies and target

Changes:

https://github.com/slackhq/python-slackclient/blob/1.0.0/CHANGELOG.md

[1] https://github.com/slackhq/python-slackclient/pull/76