Fix tox so that it can run

The following changes are introduced:

* Update the list of Python versions to test (3.9+)
* Remove the now-unused `default` and `simplejson` factors
* Switch to Poetry for dependency installation
* Switch to running pytest to execute the test suite

Author: kurtmckee

tox.ini

@@ -1,9 +1,12 @@
 [tox]
-envlist = {py37,py38,py39,py310}-{default,simplejson}
+envlist =
+    py{3.9,3.10,3.11,3.12,3.13}
 
 [testenv]
+skip_install = true
 deps =
-    -rrequirements.txt
-    -rrequirements_dev.txt
+    poetry
+commands_pre =
+    poetry install --with dev
 commands =
-    python setup.py test
+    pytest