Test via tox support for all py3.9 py3.10 and py3.11

That's python versions we care about. Add explicit tests to cover that.

setup.py

@@ -106,6 +106,11 @@ setup(
 
     classifiers = [_.strip() for _ in """\
         Development Status :: 2 - Pre-Alpha
+        Programming Language :: Python
+        Programming Language :: Python :: 3
+        Programming Language :: Python :: 3.9
+        Programming Language :: Python :: 3.10
+        Programming Language :: Python :: 3.11
         Intended Audience :: Developers
         Intended Audience :: Telecommunications Industry
         Operating System :: POSIX :: Linux\

tox.ini

+[tox]
+envlist = py39,py310,py311
+
+[testenv]
+basepython =
+    py39:   python3.9
+    py310:  python3.10
+    py311:  python3.11
+
+deps    = .[test]
+commands= {envpython} -m pytest