Update mypy requirement from <1.20.0 to <1.21.0 in /lang/py (#3718)

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
...

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Author: dependabot[bot]

lang/py/pyproject.toml

@@ -77,7 +77,7 @@ warn_unused_ignores = true
 [dependency-groups]
 dev = [
     "coverage>=7.13.4",
-    "mypy<1.20.0",
+    "mypy<1.21.0",
     "python-snappy>=0.7.3",
     "ruff>=0.15.1",
     "setuptools>=82.0.0",