fix: raise minimum Python version for type checking to 3.9 (#416)

* raise minimum Python version for mypy to Python 3.9 for compatibility with numpy 1.25.0

.pre-commit-config.yaml

@@ -7,10 +7,11 @@ repos:
     rev: v1.3.0
     hooks:
     -   id: mypy
-        name: mypy with Python 3.8
+        name: mypy with Python 3.9
         files: src/cabinetry
         additional_dependencies: ["numpy>=1.22", "boost-histogram>=1.0.1", "click>=8", "types-tabulate", "types-PyYAML"]
-        args: ["--python-version=3.8"]
+        # numpy 1.25 is no longer compatible with Python 3.8, so use Python >=3.9 for type checking
+        args: ["--python-version=3.9"]
     -   id: mypy
         name: mypy with Python 3.11
         files: src/cabinetry

pyproject.toml

@@ -31,7 +31,8 @@ warn_unreachable = true
 warn_unused_ignores = true
 strict_equality = true
 no_implicit_optional = true
-python_version = "3.8"
+# numpy 1.25 is no longer compatible with Python 3.8, so use Python >=3.9 for type checking
+python_version = "3.9"
 
 [[tool.mypy.overrides]]
 module = [