Update hikari requirement from ~=2.0.0.dev120 to ~=2.0.0.dev121

Updates the requirements on [hikari](https://github.com/hikari-py/hikari) to permit the latest version.
- [Release notes](https://github.com/hikari-py/hikari/releases)
- [Changelog](https://github.com/hikari-py/hikari/blob/master/CHANGELOG.md)
- [Commits](hikari-py/hikari@2.0.0.dev120...2.0.0.dev121)

---
updated-dependencies:
- dependency-name: hikari
  dependency-type: direct:production
...

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

requirements.txt

@@ -1 +1 @@
-hikari~=2.0.0.dev120
+hikari~=2.0.0.dev121