v3.0.0rc1

This is the first release candidate version of slack_sdk v3.0.0. The remarkable updates in this major version are:

• Newly added OAuth flow support
• Better Async/sync separation for WebClient and WebhookClient
• Renamed packages (from slack to slack_sdk) with deprecation warnings

Refer to the documents for details.

The migration guide for slackclient v2.x users is available at https://slack.dev/python-slack-sdk/v3-migration/