Secret is unnecessarily invariant, can be made covariant #12007

I noticed that the generic Secret type is unnecessarily invariant, which means that valid usages like this fail to type check:

from pydantic import Secret

def takes_secret(scalar_secret: Secret[str | int | float | bool]):
    ...

def secret_usage() -> None:
    secret: Secret[str] = Secret('my secret')
    takes_secret(secret)

Currently that last line does not type check, because the invariance prevents to accept e.g. a Secret[str] where a Secret[str | ...] is expected. Note that this restriction is unnecessary: Secret is an immutable type, both practically and conceptually. It's a typical covariant use case and there is no need to enforce invariance. The type can be made covariant similarly to how e.g. Sequence or Mapping are covariant thanks to immutability.

I can prepare a small PR for fixing it.