Bump slackclient from 2.9.3 to 2.9.4

Bumps [slackclient](https://github.com/slackapi/python-slack-sdk) from 2.9.3 to 2.9.4.
- [Release notes](https://github.com/slackapi/python-slack-sdk/releases)
- [Commits](slackapi/python-slack-sdk@v2.9.3...v2.9.4)

---
updated-dependencies:
- dependency-name: slackclient
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>