Explicitly prevent Python 3.12

To discuss: If we don't support 3.12, maybe we should enforce this here in `pyproject.toml`?

Source: <#441 (comment)>

I'm not convinced that this is the right move since it might slightly hinder testing. But if we are explicit, then this seems to me like the correct place for it.

pyproject.toml

@@ -12,7 +12,7 @@ name = "pytensor"
 dynamic = [
   'version'
 ]
-requires-python = ">=3.9"
+requires-python = ">=3.9,<3.12"
 authors = [
     {name = "pymc-devs", email = "pymc.devs@gmail.com"}
 ]