chore: extend tox.ini with Python 3.13

Author: ntamas

tox.ini

@@ -4,15 +4,16 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py38, py39, py310, py311, py312, pypy3
+envlist = py38, py39, py310, py311, py312, py313, pypy3
 
 [gh-actions]
 python =
     3.8: py38
     3.9: py39
-	3.10: py310
-	3.11: py311
-	3.12: py312
+    3.10: py310
+    3.11: py311
+    3.12: py312
+    3.13: py313
     pypy-3.7: pypy3
 
 [testenv]