Type annotations are not preserved by `declared_attr`, `hybrid_property`, and potentially other decorators

First of all, thanks a ton for SQLAlchemy -- it's been absolutely invaluable to my work.

Describe the bug

I'm using declared_attr and hybrid_property throughout one of my codebases. That codebase is automatically documented using Sphinx + sphinx-apidoc, which includes type annotations (via thing.__annotations__) when present.

This normally works just fine, but SQLAlchemy's various decorators don't preserve __annotations__. As such, various decorated APIs in my documentation lack type signatures.

Expected behavior

declared_attr and hybrid_property should preserve their underlying __annotations__ the way they currently do for __doc__.

To Reproduce

I don't have a simple test case yet, but I can try to produce one in a bit. In general, however:

1. Take a SQLAlchemy codebase that uses hybrid_property or another decorator.
2. Add type hints to the underlying (decorated) method
3. Attempt to generate Sphinx docs for the containing module
4. Check whether the generated docs include a type signature for the decorated method

Versions.

• OS: Ubuntu 20.04
• Python: CPython 3.8
• SQLAlchemy: 1.3.12
• Database: Postgres
• DBAPI: psycopg2

I'm going to take a stab at fixing this, and will open a PR if my fixes work locally. Just figured I'd open an issue to track it as well 🙂

Start with the declared_attr and hybrid_property entry points, using an annotated underlying method and checking how the decorators currently expose doc. Verify the behavior through Sphinx and sphinx-apidoc; done means the decorated APIs retain their annotations and generated documentation includes their type signatures.