Drop Python 3.9 support

At present you should be able to use Python 3.9 but

- I do not have the time to test
- I do not have the time to handle any work-arounds

So it is safest to drop explicit support.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## 0.0.31
 
+Drop support for Python 3.9 as I do not have time or energy to
+test with old versions. If this is a problem then please submit
+the necessary fixes.
+
 Internal minor clean-up of the configuration code: remove tracking
 of the model.dat file from the MANIFEST, avoid an un-used import
 from the refactoring done in 0.0.30, and move the include directory

pyproject.toml

@@ -19,15 +19,14 @@ classifiers = [
   "Intended Audience :: Science/Research",
   "License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)",
   "Programming Language :: C",
-  "Programming Language :: Python :: 3.9",
   "Programming Language :: Python :: 3.10",
   "Programming Language :: Python :: 3.11",
   "Programming Language :: Python :: 3.12",
   "Topic :: Scientific/Engineering :: Astronomy",
   "Topic :: Scientific/Engineering :: Physics"
 ]
 
-requires-python = ">=3.9"
+requires-python = ">=3.10"
 dependencies = [
   "numpy"
 ]