Update fakeredis requirement from ==2.18.* to ==2.21.* (#3899)

Updates the requirements on [fakeredis](https://github.com/cunla/fakeredis-py) to permit the latest version.
- [Release notes](https://github.com/cunla/fakeredis-py/releases)
- [Commits](https://github.com/cunla/fakeredis-py/compare/v2.18.0...v2.21.0)

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

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pyproject.toml

@@ -112,7 +112,7 @@ dev = [
   "aiohttp==3.9.*",
   "coverage",
   "coveralls",
-  "fakeredis==2.18.*",
+  "fakeredis==2.21.*",
   "flake8==6.0.*",
   "freezegun",
   "isort==5.13.*",