VType rejects new python 3.13 generated attributes `__static_attributes__` and `__firstlineno__`

[burnpanck]

A custom VType of mine that previously worked, fails on python 3.13:

TypeError: a VType can not define any class attribute except for ('__type__', '__validators__', '__help_msg__', '__error_type__', '__module__', '__qualname__', '__doc__', '__orig_bases__'). Found: {'__static_attributes__', '__firstlineno__'}

The two attributes in question (__static_attributes__ and __firstlineno__) are new in python 3.13.

I should be able to whip-up a PR quickly enough. There are a number of ways to fix that:

1. Explicitly create the correct list of acceptable attributes in a version-dependent fashion.
2. Introspecting a "golden" reference type, and taking those attributes as acceptable.
3. Blanket allowance of double-underscore attributes.

I'm tempted to suggest to go for 3 - it feels the most pythonic. What do you think?