Confine PEP 695 type annotation rewrites (`UP040`) to non-runtime contexts

[charliermarsh]

See the comment here: #6289 (comment). PEP 695 type aliases can't be used for runtime typing, so we need to limit the cases in which we flag and rewrite them. For starters, we could consider limiting to .pyi files, where it should ~always be safe. We could then extend to "private" type aliases that aren't used in runtime contexts within a file.

Alternatively, we could just turn off this rule when keep-runtime-typing is set, and leave it as-is.

[charliermarsh]

\cc @zanieb

[zanieb]

Thank for raising Henry!

Here are some notes on validity after some experimentation:

import typing

# valid
x: typing.TypeAlias = int
print(isinstance(1, x))

# valid
x: typing.TypeAlias = int | str
print(isinstance(1, x))

# valid
x: typing.TypeAlias = typing.Union[int, str]
print(isinstance(1, x))

# not valid
x: typing.TypeAlias = list[int]
print(isinstance([1,2], x))

# not valid
x: typing.TypeAlias = typing.list[int]
print(isinstance([1,2], x))

# valid
from pydantic import BaseModel
type x = int
class Foo(BaseModel):
    y: x
f = Foo(y=1)
print(f.y)

# not valid
type x = int | str
print(isinstance(1, x))

# not valid
type x = typing.Union[int, str]
print(isinstance(1, x))

It may be enough to categorize this as a "Suggested" fix.