Consider not applying PEP 604 by presence of __future__

Applying PEP 604 on non 3.10 code will crash on libraries depending on introspection of type hints using typing.get_type_hints. This should apply for libraries like https://github.com/samuelcolvin/pydantic and https://github.com/konradhalas/dacite.

Example:

>>> from __future__ import annotations
>>> import typing
>>> class Snake(typing.NamedTuple):
...     name: str | None
...
>>> typing.get_type_hints(Snake)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python@3.9/3.9.1_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/typing.py", line 1410, in get_type_hints
    value = _eval_type(value, base_globals, localns)
  File "/usr/local/Cellar/python@3.9/3.9.1_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/typing.py", line 277, in _eval_type
    return t._evaluate(globalns, localns, recursive_guard)
  File "/usr/local/Cellar/python@3.9/3.9.1_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/typing.py", line 533, in _evaluate
    eval(self.__forward_code__, globalns, localns),
  File "<string>", line 1, in <module>
TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'
class Snake(typing.NamedTuple):
    name: str | None

My proposal is to only rewrite to PEP 604 when the minimum version is 3.10.

[asottile]

#389 probably does what you want

[antonagestam]

@asottile Hey, thanks for reconsidering and fixing! 🙏