fix: run the tests in python 3.12 and 3.13 and remove snapshottest dependency (#1572)

* 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()

Author: PoByBolek

docs/testing/index.rst

@@ -69,43 +69,3 @@ You can also add extra keyword arguments to the ``execute`` method, such as
                 'hey': 'hello Peter!'
             }
         }
-
-
-Snapshot testing
-~~~~~~~~~~~~~~~~
-
-As our APIs evolve, we need to know when our changes introduce any breaking changes that might break
-some of the clients of our GraphQL app.
-
-However, writing tests and replicating the same response we expect from our GraphQL application can be a
-tedious and repetitive task, and sometimes it's easier to skip this process.
-
-Because of that, we recommend the usage of `SnapshotTest <https://github.com/syrusakbary/snapshottest/>`_.
-
-SnapshotTest lets us write all these tests in a breeze, as it automatically creates the ``snapshots`` for us
-the first time the test are executed.
-
-
-Here is a simple example on how our tests will look if we use ``pytest``:
-
-.. code:: python
-
-    def test_hey(snapshot):
-        client = Client(my_schema)
-        # This will create a snapshot dir and a snapshot file
-        # the first time the test is executed, with the response
-        # of the execution.
-        snapshot.assert_match(client.execute('''{ hey }'''))
-
-
-If we are using ``unittest``:
-
-.. code:: python
-
-    from snapshottest import TestCase
-
-    class APITestCase(TestCase):
-        def test_api_me(self):
-            """Testing the API for /me"""
-            client = Client(my_schema)
-            self.assertMatchSnapshot(client.execute('''{ hey }'''))