Replace slackclient with slack-sdk for Slack action

[codecov[bot]]

Codecov Report

Merging #338 (1f20bf7) into master (7562a67) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #338   +/-   ##
=======================================
  Coverage   74.04%   74.04%           
=======================================
  Files          68       68           
  Lines        3028     3028           
  Branches      465      465           
=======================================
  Hits         2242     2242           
  Misses        723      723           
  Partials       63       63           

Impacted Files	Coverage Δ
spidermon/contrib/actions/slack/__init__.py	56.63% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 7562a67...1f20bf7. Read the comment docs.

[rennerocha]

I tested locally and the changes worked fine. Slack messages are sent as before.

[rennerocha]

Fix #313