* actually run the tests in python 3.12 and 3.13
* remove snapshottest from the example tests
so that the tests pass in 3.12 and 3.13 again
* remove the section about snapshot testing from the testing docs
because the snapshottest package doesn't work on Python 3.12 and above
* fix assertion for badly formed JSON input on Python 3.13
* fix deprecation warning about datetime.utcfromtimestamp()