Assigning functions to orm attributes

[MaicoTimmerman]

Describe the bug
Introduced by #40, sql.functions are no longer able to be assigned to ORM attributes. I couldn't find documented cases of directly assigning functions to attributes. Is it even a supported use-case? Or should update() always be used.

Expected behavior
Type checking to pass, as it works at runtime

To Reproduce

from sqlalchemy import Column, DateTime
from sqlalchemy import Integer
from sqlalchemy.orm import registry
from sqlalchemy.sql.functions import now

mapper_registry: registry = registry()

@mapper_registry.mapped
class A:
    __tablename__ = "a"
    id = Column(Integer, primary_key=True)
    date_time = Column(DateTime())

a = A()
a.date_time = now()

Error

test/ext/mypy/files/function_now.py:15: error: Incompatible types in assignment (expression has type "now[<nothing>]", variable has type "Optional[datetime]")```

Have a nice day!

[zzzeek]

that's definitely supported:

https://docs.sqlalchemy.org/en/14/orm/persistence_techniques.html#embedding-sql-insert-update-expressions-into-a-flush