Python: Keep runtime typing

[Fokko]

Pyupgrade would update Dict[str, Any] to dict[str, Any] which
isn't allowed in Python 3.8, breaking the tests with an obscure error:

ImportError while loading conftest '/Users/fokkodriesprong/Desktop/iceberg/python/tests/conftest.py'.
../conftest.py:34: in <module>
    from iceberg import schema
../../src/iceberg/schema.py:47: in <module>
    class Schema(IcebergBaseModel):
pydantic/main.py:188: in pydantic.main.ModelMetaclass.__new__
    ???
pydantic/typing.py:419: in pydantic.typing.resolve_annotations
    ???
/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/typing.py:270: in _eval_type
    return t._evaluate(globalns, localns)
/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/typing.py:518: in _evaluate
    eval(self.__forward_code__, globalns, localns),
E   TypeError: 'type' object is not subscriptable

Therefore we should disable them (until we go to Python 3.9 :)

[Fokko]

It seems to be something specific to my branch