Upgrade slack-sdk version

Author: seratch

setup.py

@@ -14,7 +14,7 @@
     long_description = fh.read()
 
 test_dependencies = [
-    "slack_sdk>=3.19,<4",
+    "slack_sdk>=3.21.3,<4",
     "pytest>=6,<7",
     "pytest-cov>=3,<4",
     "black==22.8.0",  # 22.10.0 or newer no longer exist for Python 3.6