CI: test on CPython 3.15.0 #111

CPython 3.15.0 is final, and the ecosystem's interpreter is 3.15 (free-threaded). Tests run only on 3.13 today. Run the no-binder job on 3.13 and 3.15 (3.13 is the floor the package still claims via requires-python >= 3.11 users), and the with-binder job on 3.15. Done when both jobs pass on 3.15.