text: Title: Incorrect type hint for WebClient#files_upload file argument
  Body: After #771 was merged, the `files_upload` function signature still does not accept a `bytes` type object: https://github.com/slackapi/python-slack-sdk/blob/main/slack_sdk/web/client.py#L1458-L1460
  
  ```
      def files_upload(
          self, *, file: Union[str, IOBase] = None, content: str = None, **kwargs
      ) -> SlackResponse:
  ```
  So the OP example produces a mypy error: `Argument "file" to "files_upload" of "WebClient" has incompatible type "bytes"; expected "Union[str, IOBase, None]"`
  
  _Originally posted by @gburek-fastly in https://github.com/slackapi/python-slack-sdk/issues/770#issuecomment-749221824_