Constrain development tools by Python version

HACKING.txt

@@ -68,6 +68,23 @@ Install test dependencies::
 
    uv sync --locked
 
+Development dependency compatibility
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The dependency lock contains temporary constraints which preserve the full
+runtime test matrix without making development tools part of the package's
+runtime requirements:
+
+- The ``test`` group selects ``pytest<9`` on Python 3.9 and ``pytest>=9`` on
+  newer interpreters because pytest 9 requires Python 3.10 or later. Replace
+  these marked requirements with an unconstrained ``pytest`` requirement when
+  Python 3.9 support is dropped.
+- The ``docs``, ``dev``, ``lint``, and ``security`` groups declare Python 3.11
+  or later in ``[tool.uv.dependency-groups]``. These declarations prevent
+  tooling that has dropped Python 3.9 or 3.10 from narrowing nagiosplugin's
+  runtime support. They become redundant and may be removed when the package's
+  minimum supported Python version reaches 3.11.
+
 After doing so, run the unit tests::
 
    uv run tox

pyproject.toml

@@ -97,13 +97,17 @@ security = [
     "pip-audit",
 ]
 test = [
-    "pytest",
+    # pytest 9 requires Python 3.10, but the test matrix still includes 3.9.
+    "pytest<9; python_version < '3.10'",
+    "pytest>=9; python_version >= '3.10'",
     "pytest-cov",
 ]
 
 [tool.uv.dependency-groups]
 docs = {requires-python = ">=3.11"}
 dev = {requires-python = ">=3.11"}
+lint = {requires-python = ">=3.11"}
+security = {requires-python = ">=3.11"}
 
 [tool.pylint]
 py-version=3.9