Fix #850 by upgrading slack-sdk version to the latest (#852)

Author: seratch

setup.py

@@ -58,7 +58,7 @@
     ),
     include_package_data=True,  # MANIFEST.in
     install_requires=[
-        "slack_sdk>=3.18.5,<4",
+        "slack_sdk>=3.20.2,<4",
     ],
     setup_requires=["pytest-runner==5.2"],
     tests_require=async_test_dependencies,