Update discord-py requirement from ~=2.1.0 to ~=2.4.0

Updates the requirements on [discord-py](https://github.com/Rapptz/discord.py) to permit the latest version.
- [Commits](Rapptz/discord.py@v2.1.0...v2.4.0)

---
updated-dependencies:
- dependency-name: discord-py
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -1,4 +1,4 @@
-discord.py~=2.1.0
+discord.py~=2.4.0
 emoji~=2.0.0
 pymongo~=3.13.0
 pymongo-stubs~=0.2.0