Support conditional definitions in dataclasses, etc.

**Feature**

Support Python version and other checks that can be evaluated statically in bodies of dataclasses, named tuples, and other cases discussed in https://github.com/python/typing-council/issues/58.

Example that should work:
```py
import sys

from typing import NamedTuple

class NT(NamedTuple):
    x: int
    if sys.version_info >= (3, 12):
        y: str
```