Use OAuth for API

[rho333]

Is your feature request related to a problem? Please describe.
The current API client implementation uses Slack's legacy tokens, which are deprecated, less secure and likely to be removed in the near-medium term. In addition, access to legacy tokens requires a Slack admin approval, which adds a significant obstacle to using this tool.

Describe the solution you'd like
Change the implementation to use OAuth.

Describe alternatives you've considered
My personal preference is to adopt the use of Slack's official Python API wrapper.

Additional context
N/A