Extend & Unify Deserialization Testing #4635

This issue is a consequence of #4634 and #4617 (comment).

Currently, for each TO class we manually implement test_de_json and some classes also implement test_de_json_required and test_de_json_localization. Personally I'm okay with the explicit repitition since automating tests across several classes can be error prone itself (see #4593 and also the convoluted logic in https://github.com/python-telegram-bot/python-telegram-bot/blob/v21.9/tests/auxil/bot_method_checks.py).
However, the findings above show that we should try to ensure that we test all uses cases in all classes, that being

1. deserialization works if optional arguments are missing
2. deserialization works if additional arguments are passed (backward compatibility for new arguments)
3. To be discussed¹: deserialization works if required arguments are missing (forward compatibility for TG removing arguments)
4. If required for that class: deserialization correctly localizes datetimes

I can see at least three options for ensuring that a test class tests all required use cases

1. Introducing helper methods that do all that and calling them in the test classes. Adantvage: Less repetition. Disatvantage: Still need to ensure that the helper methods are called and also the downside of increased test complexity.
2. Introduce abstract base class for TO-Class tests with abstract methods test_de_json, test_de_json_required, test_de_json_locatization(?) and possibly others. This should force subclasses to implement all relevant tests. Advantage: Enforced unified setup without added test complexity. Disadvantage: Still need to check that the ABC is used. Could be done with a meta-test.
3. Add a meta-test that checks simply that methods with the corresponding names exist. Advantage: Enforced unified setup without much added test complexity. Disadvantage: Less explicit than 2.

At first glance, I'm in favor of 2.

@harshil21 @Poolitzer do you have any preferences?

¹ So far, Telegram has always made the removal of fields backward compatible (with the exception of thumb back in 2015), see

• https://core.telegram.org/bots/api-changelog (search for "backward")
• Introduce RichText type tdlib/telegram-bot-api#526 (comment) (§2)

Of course, as per usual, Telegram doesn't document such things.