fix: pin mypy below 2.0 for Python 3.9

changelog.d/20260802_issue_44_mypy_python_39.md

@@ -0,0 +1,4 @@
+### Fixed
+
+- Keep the development mypy version below 2.0 while the template supports and
+  type-checks against Python 3.9.

pyproject.toml

@@ -28,7 +28,7 @@ classifiers = [
 [project.optional-dependencies]
 dev = [
     "ruff>=0.8.0,<0.9",
-    "mypy>=1.13.0",
+    "mypy>=1.13.0,<2.0",
     "pytest>=8.3.0",
     "pytest-asyncio>=0.24.0",
     "pytest-cov>=6.0.0",

tests/test_project_metadata.py

@@ -0,0 +1,26 @@
+"""Regression tests for project package metadata."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from packaging.requirements import Requirement
+
+ROOT = Path(__file__).resolve().parents[1]
+
+
+def dev_requirement(name: str) -> Requirement:
+    """Return one requirement from the project's ``dev`` dependency group."""
+    pyproject = (ROOT / "pyproject.toml").read_text(encoding="utf-8")
+    dev_group = pyproject.split("dev = [", maxsplit=1)[1].split("]", maxsplit=1)[0]
+    requirement_lines = (line.strip().strip('",') for line in dev_group.splitlines())
+    requirements = (Requirement(line) for line in requirement_lines if line)
+    return next(requirement for requirement in requirements if requirement.name == name)
+
+
+def test_mypy_dependency_supports_configured_python_target() -> None:
+    """The dev extra must not install mypy versions that reject Python 3.9."""
+    mypy = dev_requirement("mypy")
+
+    assert "1.13.0" in mypy.specifier
+    assert "2.0" not in mypy.specifier