Chore(deps-dev): Update mypy requirement from <1.20 to <1.21

Updates the requirements on [mypy](https://github.com/python/mypy) to permit the latest version.
- [Changelog](https://github.com/python/mypy/blob/master/CHANGELOG.md)
- [Commits](python/mypy@v0.1.0...v1.20.0)

---
updated-dependencies:
- dependency-name: mypy
  dependency-version: 1.20.0
  dependency-type: direct:development
...

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

Author: dependabot[bot]

pyproject.toml

@@ -58,7 +58,7 @@ dependencies = [
 dev = [
     "certifi",
     "coverage",
-    "mypy<1.20",
+    "mypy<1.21",
     "pytest<10",
     "pytz",
     "ruff<0.16",