Uprev tox.ini to test python 3.13

Also corrected some of the package classifiers to indicate that this
python library no longer supports python 2. It didn't even with the
older 0.20.* Pint, though Pint itself now only supports python 3.9+

Author: Brendan Whitfield

setup.py

@@ -16,8 +16,12 @@
         "Operating System :: POSIX :: Linux",
         "License :: OSI Approved :: GNU General Public License v2 (GPLv2)",
         "Topic :: System :: Monitoring",
-        "Programming Language :: Python :: 2",
         "Programming Language :: Python :: 3",
+        "Programming Language :: Python :: 3.9",
+        "Programming Language :: Python :: 3.10",
+        "Programming Language :: Python :: 3.11",
+        "Programming Language :: Python :: 3.12",
+        "Programming Language :: Python :: 3.13",
         "Development Status :: 3 - Alpha",
         "Topic :: System :: Logging",
         "Intended Audience :: Developers",

tox.ini

@@ -1,14 +1,15 @@
 [tox]
 envlist =
-    py{38,39,310,311},
+    py{39,310,311,312,313},
     coverage
 
 [gh-actions]
 python =
-    3.8: py38
     3.9: py39
     3.10: py310
     3.11: py311
+    3.12: py312
+    3.13: py313
 
 [testenv]
 usedevelop = true