Update to Python 3.10 as minimum (3.9 is EOL)

Also write down Python support policy.

Author: stefanv

.github/workflows/test.yml

@@ -20,7 +20,7 @@ jobs:
   test_spin:
     strategy:
       matrix:
-        python_version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14-dev"]
+        python_version: ["3.10", "3.11", "3.12", "3.13", "3.14"]
         os: [ubuntu-latest, windows-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:

README.md

@@ -296,6 +296,9 @@ nox -s test -- -v
 nox -s test -- -v spin/tests/test_meson.py
 ```
 
+`spin` takes a slightly more conservative approach than [SPEC 0](https://scientific-python.org/specs/spec-0000/), and
+supports all non-EOL versions of Python.
+
 ## History
 
 The `dev.py` tool was [proposed for SciPy](https://github.com/scipy/scipy/issues/15489) by Ralf Gommers and [implemented](https://github.com/scipy/scipy/pull/15959) by Sayantika Banik, Eduardo Naufel Schettino, and Ralf Gommers (also see [Sayantika's blog post](https://labs.quansight.org/blog/the-evolution-of-the-scipy-developer-cli)).

example_pkg_src/pyproject.toml

@@ -1,7 +1,7 @@
 [project]
 name = "example_pkg"
 version = "0.0dev0"
-requires-python = ">=3.9"
+requires-python = ">=3.10"
 description = "spin Example Package"
 
 [build-system]

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "spin"
-requires-python = ">=3.9"
+requires-python = ">=3.10"  # Oldest non-EOL Python here
 description = "Developer tool for scientific Python libraries"
 readme = "README.md"
 license = {file = "LICENSE"}
@@ -15,7 +15,6 @@ classifiers = [
   "License :: OSI Approved :: BSD License",
   "Operating System :: OS Independent",
   "Programming Language :: Python :: 3",
-  "Programming Language :: Python :: 3.9",
   "Programming Language :: Python :: 3.10",
   "Programming Language :: Python :: 3.11",
   "Programming Language :: Python :: 3.12",