Bump discord-py from 2.5.2 to 2.6.4

Bumps [discord-py](https://github.com/Rapptz/discord.py) from 2.5.2 to 2.6.4.
- [Commits](Rapptz/discord.py@v2.5.2...v2.6.4)

---
updated-dependencies:
- dependency-name: discord-py
  dependency-version: 2.6.4
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]